(Quick Reference)

5. Object Relational Mapping (GORM) - Reference Documentation

Authors: Graeme Rocher, Peter Ledbrook, Marc Palmer, Jeff Brown, Luke Daley, Burt Beckwith

Version: 1.4.0.M1

5. Object Relational Mapping (GORM)

Domain classes are core to any business application. They hold state about business processes and hopefully also implement behavior. They are linked together through relationships, either one-to-one or one-to-many.

GORM is Grails' object relational mapping (ORM) implementation. Under the hood it uses Hibernate 3 (an extremely popular and flexible open source ORM solution) but because of the dynamic nature of Groovy, the fact that it supports both static and dynamic typing, and the convention of Grails there is less configuration involved in creating Grails domain classes.

You can also write Grails domain classes in Java. See the section on Hibernate Integration for how to write Grails domain classes in Java but still use dynamic persistent methods. Below is a preview of GORM in action:

def book = Book.findByTitle("Groovy in Action")

book .addToAuthors(name:"Dierk Koenig") .addToAuthors(name:"Guillaume LaForge") .save()

5.1 Quick Start Guide

A domain class can be created with the create-domain-class command:

grails create-domain-class helloworld.Person

If no package is specified with the create-domain-class script, Grails automatically uses the application name as the package name.

This will create a class at the location grails-app/domain/helloworld/Person.groovy such as the one below:

package helloworld
class Person {
}

If you have the dbCreate property set to "update", "create" or "create-drop" on your DataSource, Grails will automatically generated/modify the database tables for you.

You can customize the class by adding properties:

class Person {
    String name
    Integer age
    Date lastVisit
}

Once you have a domain class try and manipulate it via the shell or console by typing:

grails console

This loads an interactive GUI where you can type Groovy commands.

5.1.1 Basic CRUD

Try performing some basic CRUD (Create/Read/Update/Delete) operations.

Create

To create a domain class use the Groovy new operator, set its properties and call save:

def p = new Person(name:"Fred", age:40, lastVisit:new Date())
p.save()

The save method will persist your class to the database using the underlying Hibernate ORM layer.

Read

Grails transparently adds an implicit id property to your domain class which you can use for retrieval:

def p = Person.get(1)
assert 1 == p.id

This uses the get method that expects a database identifier to read the Person object back from the db. You can also load an object in a read-only state by using the read method:

def p = Person.read(1)

In this case the underlying Hibernate engine will not do any dirty checking and the object will not be persisted. Note that if you explicitly call the save method then the object is placed back into a read-write state.

In addition, you can also load an proxy for an instance by using the load method:

def p = Person.load(1)

This incurs no database access until a method other than getId() is called. Hibernate then initializes the proxied instance, or throws an exception if no record is found for the specified id.

Update

To update an instance, set some properties and then simply call save again:

def p = Person.get(1)
p.name = "Bob"
p.save()

Delete

To delete an instance use the delete method:

def p = Person.get(1)
p.delete()

5.2 Domain Modelling in GORM

When building Grails applications you have to consider the problem domain you are trying to solve. For example if you were building an Amazon bookstore you would be thinking about books, authors, customers and publishers to name a few.

These are modeled in GORM as Groovy classes so a Book class may have a title, a release date, an ISBN number and so on. The next few sections show how to model the domain in GORM.

To create a domain class you can run the create-domain-class target as follows:

grails create-domain-class Book

The result will be a class at grails-app/domain/Book.groovy:

class Book {
}

If you wish to use packages you can move the Book.groovy class into a sub directory under the domain directory and add the appropriate package declaration as per Groovy (and Java's) packaging rules.

The above class will map automatically to a table in the database called book (the same name as the class). This behaviour is customizable through the ORM Domain Specific Language

Now that you have a domain class you can define its properties as Java types. For example:

class Book {
    String title
    Date releaseDate
    String ISBN
}

Each property is mapped to a column in the database, where the convention for column names is all lower case separated by underscores. For example releaseDate maps onto a column release_date. The SQL types are auto-detected from the Java types, but can be customized via Constraints or the ORM DSL.

5.2.1 Association in GORM

Relationships define how domain classes interact with each other. Unless specified explicitly at both ends, a relationship exists only in the direction it is defined.

5.2.1.1 Many-to-one and one-to-one

A many-to-one relationship is the simplest kind, and is defined trivially using a property of the type of another domain class. Consider this example:

Example A

class Face {
    Nose nose
}
class Nose {
}

In this case we have a unidirectional many-to-one relationship from Face to Nose. To make this relationship bidirectional define the other side as follows:

Example B

class Face {
    Nose nose
}
class Nose {
    static belongsTo = [face:Face]
}

In this case we use the belongsTo setting to say that Nose "belongs to" Face. The result of this is that we can create a Face, attach a Nose instance to it and when we save or delete the Face instance, GORM will save or delete the Nose. In other words, saves and deletes will cascade from Face to the associated Nose:

new Face(nose:new Nose()).save()

The example above will save both face and nose. Note that the inverse is not true and will result in an error due to a transient Face:

new Nose(face:new Face()).save() // will cause an error

Now if we delete the Face instance, the Nose will go too:

def f = Face.get(1)
f.delete() // both Face and Nose deleted

To make the relationship a true one-to-one, use the hasOne property on the owning side, e.g. Face:

Example C

class Face {
    static hasOne = [nose:Nose]
}
class Nose {
    Face face
}

Note that using this property puts the foreign key on the inverse table to the previous example, so in this case the foreign key column is stored in the nose table inside a column called face_id. Also, hasOne only works with bidirectional relationships.

Finally, it's a good idea to add a unique constraint on one side of the one-to-one relationship:

class Face {
    static hasOne = [nose:Nose]

static constraints = { nose unique: true } }

class Nose { Face face }

5.2.1.2 One-to-many

A one-to-many relationship is when one class, example Author, has many instances of a another class, example Book. With Grails you define such a relationship with the hasMany setting:

class Author {
    static hasMany = [ books : Book ]

String name } class Book { String title }

In this case we have a unidirectional one-to-many. Grails will, by default, map this kind of relationship with a join table.

The ORM DSL allows mapping unidirectional relationships using a foreign key association instead

Grails will automatically inject a property of type java.util.Set into the domain class based on the hasMany setting. This can be used to iterate over the collection:

def a = Author.get(1)

a.books.each { println it.title }

The default fetch strategy used by Grails is "lazy", which means that the collection will be lazily initialized. This can lead to the n+1 problem if you are not careful.

If you need "eager" fetching you can use the ORM DSL or specify eager fetching as part of a query

The default cascading behaviour is to cascade saves and updates, but not deletes unless a belongsTo is also specified:

class Author {
    static hasMany = [ books : Book ]

String name } class Book { static belongsTo = [author:Author] String title }

If you have two properties of the same type on the many side of a one-to-many you have to use mappedBy to specify which the collection is mapped:

class Airport {
    static hasMany = [flights:Flight]
    static mappedBy = [flights:"departureAirport"]
}
class Flight {
    Airport departureAirport
    Airport destinationAirport
}

This is also true if you have multiple collections that map to different properties on the many side:

class Airport {
    static hasMany = [outboundFlights:Flight, inboundFlights:Flight]
    static mappedBy = [outboundFlights:"departureAirport", inboundFlights:"destinationAirport"]
}
class Flight {
    Airport departureAirport
    Airport destinationAirport
}

5.2.1.3 Many-to-many

Grails supports many-to-many relationships by defining a hasMany on both sides of the relationship and having a belongsTo on the owned side of the relationship:

class Book {
    static belongsTo = Author
    static hasMany = [authors:Author]
    String title
}
class Author {
    static hasMany = [books:Book]
    String name
}

Grails maps a many-to-many using a join table at the database level. The owning side of the relationship, in this case Author, takes responsibility for persisting the relationship and is the only side that can cascade saves across.

For example this will work and cascade saves:

new Author(name:"Stephen King")
        .addToBooks(new Book(title:"The Stand"))
        .addToBooks(new Book(title:"The Shining"))
        .save()

However the below will only save the Book and not the authors!

new Book(name:"Groovy in Action")
        .addToAuthors(new Author(name:"Dierk Koenig"))
        .addToAuthors(new Author(name:"Guillaume Laforge"))
        .save()

This is the expected behaviour as, just like Hibernate, only one side of a many-to-many can take responsibility for managing the relationship.

Grails' Scaffolding feature does not currently support many-to-many relationship and hence you must write the code to manage the relationship yourself

5.2.1.4 Basic Collection Types

As well as associations between different domain classes, GORM also supports mapping of basic collection types. For example, the following class creates a nicknames association that is a Set of String instances:

class Person {
    static hasMany = [nicknames:String]
}

GORM will map an association like the above using a join table. You can alter various aspects of how the join table is mapped using the joinTable argument:

class Person {
    static hasMany = [nicknames:String]

static mapping = { hasMany joinTable: [name: 'bunch_o_nicknames', key: 'person_id', column: 'nickname', type: "text"] } }

The example above will map to a table that looks like the following:

bunch_o_nicknames Table

---------------------------------------------
| person_id         |     nickname          |
---------------------------------------------
|   1               |      Fred             |
---------------------------------------------

5.2.2 Composition in GORM

As well as association, Grails supports the notion of composition. In this case instead of mapping classes onto separate tables a class can be "embedded" within the current table. For example:

class Person {
    Address homeAddress
    Address workAddress
    static embedded = ['homeAddress', 'workAddress']
}
class Address {
    String number
    String code
}

The resulting mapping would looking like this:

If you define the Address class in a separate Groovy file in the grails-app/domain directory you will also get an address table. If you don't want this to happen use Groovy's ability to define multiple classes per file and include the Address class below the Person class in the grails-app/domain/Person.groovy file

5.2.3 Inheritance in GORM

GORM supports inheritance both from abstract base classes and concrete persistent GORM entities. For example:

class Content {
     String author
}
class BlogEntry extends Content {
    URL url
}
class Book extends Content {
    String ISBN
}
class PodCast extends Content {
    byte[] audioStream
}

In the above example we have a parent Content class and then various child classes with more specific behaviour.

Considerations

At the database level Grails by default uses table-per-hierarchy mapping with a discriminator column called class so the parent class (Content) and its sub classes (BlogEntry, Book etc.), share the same table.

Table-per-hierarchy mapping has a down side in that you cannot have non-nullable properties with inheritance mapping. An alternative is to use table-per-subclass which can be enabled via the ORM DSL

However, excessive use of inheritance and table-per-subclass can result in poor query performance due to the excessive use of join queries. In general our advice is if you're going to use inheritance, don't abuse it and don't make your inheritance hierarchy too deep.

Polymorphic Queries

The upshot of inheritance is that you get the ability to polymorphically query. For example using the list method on the Content super class will return all sub classes of Content:

def content = Content.list() // list all blog entries, books and pod casts
content = Content.findAllByAuthor('Joe Bloggs') // find all by author

def podCasts = PodCast.list() // list only pod casts

5.2.4 Sets, Lists and Maps

Sets of objects

By default when you define a relationship with GORM it is a java.util.Set which is an unordered collection that cannot contain duplicates. In other words when you have:

class Author {
   static hasMany = [books:Book]
}

The books property that GORM injects is a java.util.Set. The problem with this is there is no ordering when accessing the collection, which may not be what you want. To get custom ordering you can say that the set is a SortedSet:

class Author {
   SortedSet books
   static hasMany = [books:Book]
}

In this case a java.util.SortedSet implementation is used which means you have to implement java.lang.Comparable in your Book class:

class Book implements Comparable {
   String title
   Date releaseDate = new Date()

int compareTo(obj) { releaseDate.compareTo(obj.releaseDate) } }

The result of the above class is that the Book instances in the books collections of the Author class will be ordered by their release date.

Lists of objects

If you simply want to be able to keep objects in the order which they were added and to be able to reference them by index like an array you can define your collection type as a List:

class Author {
   List books
   static hasMany = [books:Book]
}

In this case when you add new elements to the books collection the order is retained in a sequential list indexed from 0 so you can do:

author.books[0] // get the first book

The way this works at the database level is Hibernate creates a books_idx column where it saves the index of the elements in the collection in order to retain this order at the db level.

When using a List, elements must be added to the collection before being saved, otherwise Hibernate will throw an exception (org.hibernate.HibernateException: null index column for collection):

// This won't work!
def book = new Book(title: 'The Shining')
book.save()
author.addToBooks(book)

// Do it this way instead. def book = new Book(title: 'Misery') author.addToBooks(book) author.save()

Maps of Objects

If you want a simple map of string/value pairs GORM can map this with the following:

class Author {
   Map books // map of ISBN:book names
}

def a = new Author() a.books = ["1590597583":"Grails Book"] a.save()

In this case the key and value of the map MUST be strings.

If you want a Map of objects then you can do this:

class Book {
  Map authors
  static hasMany = [authors:Author]
}

def a = new Author(name:"Stephen King")

def book = new Book() book.authors = [stephen:a] book.save()

The static hasMany property defines the type of the elements within the Map. The keys for the map must be strings.

A Note on Collection Types and Performance

The Java Set type is a collection that doesn't allow duplicates. In order to ensure uniqueness when adding an entry to a Set association Hibernate has to load the entire associations from the database. If you have a large numbers of entries in the association this can be costly in terms of performance.

The same behavior is required for List types, since Hibernate needs to load the entire association in-order to maintain order. Therefore it is recommended that if you anticipate a large numbers of records in the association that you make the association bidirectional so that the link can be created on the inverse side. For example consider the following code:

def book = new Book(title:"New Grails Book")
def author = Author.get(1)
book.author = author
book.save()

In this example the association link is being created by the child (Book) and hence it is not necessary to manipulate the collection directly resulting in fewer queries and more efficient code. Given an Author with a large number of associated Book instances if you were to write code like the following you would see an impact on performance:

def book = new Book(title:"New Grails Book")
def author = Author.get(1)
author.addToBooks(book)
author.save()

5.3 Persistence Basics

A key thing to remember about Grails is that under the surface Grails is using Hibernate for persistence. If you are coming from a background of using ActiveRecord or iBatis Hibernate's "session" model may feel a little strange.

Essentially, Grails automatically binds a Hibernate session to the currently executing request. This allows you to use the save and delete methods as well as other GORM methods transparently.

5.3.1 Saving and Updating

An example of using the save method can be seen below:

def p = Person.get(1)
p.save()

A major difference with Hibernate is when you call save it does not necessarily perform any SQL operations at that point. Hibernate typically batches up SQL statements and executes them at the end. This is typically done for you automatically by Grails, which manages your Hibernate session.

There are occasions, however, when you may want to control when those statements are executed or, in Hibernate terminology, when the session is "flushed". To do so you can use the flush argument to the save method:

def p = Person.get(1)
p.save(flush:true)

Note that in this case all pending SQL statements including previous saves will be synchronized with the db. This also allows you to catch any exceptions thrown, which is typically useful in highly concurrent scenarios involving optimistic locking:

def p = Person.get(1)
try {
    p.save(flush:true)
}
catch(Exception e) {
    // deal with exception
}

Another thing to bear in mind is that Grails validates a domain instance every time you save it. If that validation fails the domain instance will not be persisted to the database. By default, save() will simply return null in this case, but if you would prefer it to throw an exception you can use the failOnError argument:

def p = Person.get(1)
try {
    p.save(failOnError: true)
}
catch (Exception e) {
    // deal with exception
}

You can even change the default behaviour via a setting in Config.groovy, as described in the section on configuration. Just remember that when you are saving domain instances that have been bound with data provided by the user, the likelihood of validation exceptions is quite high and you won't want those exceptions propagating to the end user.

You can find out more about the subtleties of saving data in this article - a must read!

5.3.2 Deleting Objects

An example of the delete method can be seen below:

def p = Person.get(1)
p.delete()

By default Grails will use transactional write behind to perform the delete, if you want to perform the delete in place then you can use the flush argument:

def p = Person.get(1)
p.delete(flush:true)

Using the flush argument will also allow you to catch any errors that may potentially occur during a delete. A common error that may occur is if you violate a database constraint, although this is normally down to a programming or schema error. The following example shows how to catch a DataIntegrityViolationException that is thrown when you violate the database constraints:

def p = Person.get(1)

try { p.delete(flush:true) } catch(org.springframework.dao.DataIntegrityViolationException e) { flash.message = "Could not delete person ${p.name}" redirect(action:"show", id:p.id) }

Note that Grails does not supply a deleteAll method as deleting data is discouraged and can often be avoided through boolean flags/logic.

If you really need to batch delete data you can use the executeUpdate method to do batch DML statements:

Customer.executeUpdate("delete Customer c where c.name = :oldName", [oldName:"Fred"])

5.3.3 Understanding Cascading Updates and Deletes

It is critical that you understand how cascading updates and deletes work when using GORM. The key part to remember is the belongsTo setting which controls which class "owns" a relationship.

Whether it is a one-to-one, one-to-many or many-to-many, defining belongsTo will result in updates cascading from the owning class to its dependant (the other side of the relationship), and for many-/one-to-one and one-to-many relationships deletes will also cascade.

If you do not define belongsTo then no cascades will happen and you will have to manually save each object (except in the case of the one-to-many, in which case saves will cascade automatically if a new instance is in a hasMany collection).

Here is an example:

class Airport {
    String name
    static hasMany = [flights:Flight]
}

class Flight { String number static belongsTo = [airport:Airport] }

If I now create an Airport and add some Flights to it I can save the Airport and have the updates cascaded down to each flight, hence saving the whole object graph:

new Airport(name:"Gatwick")
        .addToFlights(new Flight(number:"BA3430"))
        .addToFlights(new Flight(number:"EZ0938"))
        .save()

Conversely if I later delete the Airport all Flights associated with it will also be deleted:

def airport = Airport.findByName("Gatwick")
airport.delete()

However, if I were to remove belongsTo then the above cascading deletion code would not work. To understand this better take a look at the summaries below that describe the default behaviour of GORM with regards to specific associations. Also read part 2 of the GORM Gotchas series of articles to get a deeper understanding of relationships and cascading.

Bidirectional one-to-many with belongsTo

class A { static hasMany = [bees:B] }
class B { static belongsTo = [a:A] }

In the case of a bidirectional one-to-many where the many side defines a belongsTo then the cascade strategy is set to "ALL" for the one side and "NONE" for the many side.

Unidirectional one-to-many

class A { static hasMany = [bees:B] }
class B {  }

In the case of a unidirectional one-to-many where the many side defines no belongsTo then the cascade strategy is set to "SAVE-UPDATE".

Bidirectional one-to-many, no belongsTo

class A { static hasMany = [bees:B] }
class B { A a }

In the case of a bidirectional one-to-many where the many side does not define a belongsTo then the cascade strategy is set to "SAVE-UPDATE" for the one side and "NONE" for the many side.

Unidirectional one-to-one with belongsTo

class A {  }
class B { static belongsTo = [a:A] }

In the case of a unidirectional one-to-one association that defines a belongsTo then the cascade strategy is set to "ALL" for the owning side of the relationship (A->B) and "NONE" from the side that defines the belongsTo (B->A)

Note that if you need further control over cascading behaviour, you can use the ORM DSL.

5.3.4 Eager and Lazy Fetching

Associations in GORM are by default lazy. This is best explained by example:

class Airport {
    String name
    static hasMany = [flights:Flight]
}
class Flight {
    String number
    Location destination
    static belongsTo = [airport:Airport]
}
class Location {
    String city
    String country
}

Given the above domain classes and the following code:

def airport = Airport.findByName("Gatwick")
airport.flights.each {
    println it.destination.city
}

GORM will execute a single SQL query to fetch the Airport instance, another to get its flights, and then 1 extra query for each iteration over the flights association to get the current flight's destination. In other words you get N+1 queries (if you exclude the original one to get the airport).

Configuring Eager Fetching

An alternative approach that avoids the N+1 queries is to use eager fetching, which can be specified as follows:

class Airport {
    String name
    static hasMany = [flights:Flight]
    static mapping = {
        flights lazy: false
    }
}

In this case the flights association will be loaded at the same time as its Airport instance, although a second query will be executed to fetch the collection. You can also use fetch: 'join' instead of lazy: false , in which case GORM will only execute a single query to get the airports and their flights. This works well for single-ended associations, but you need to be careful with one-to-manys. Queries will work as you'd expect right up to the moment you add a limit to the number of results you want. At that point, you will likely end up with fewer results than you were expecting. The reason for this is quite technical but ultimately the problem arises from GORM using a left outer join.

So, the recommendation is currently to use fetch: 'join' for single-ended associations and lazy: false for one-to-manys.

Be careful how and where you use eager loading because you could load your entire database into memory with too many eager associations. You can find more information on the mapping options in the section on the ORM DSL.

Using Batch Fetching

Although eager fetching is appropriate for some cases, it is not always desirable. If you made everything eager you could quite possibly load your entire database into memory resulting in performance and memory problems. An alternative to eager fetching is to use batch fetching. Essentially, you can configure Hibernate to lazily fetch results in "batches". For example:

class Airport {
    String name
    static hasMany = [flights:Flight]
    static mapping = {
        flights batchSize:10
    }
}

In this case, due to the batchSize argument, when you iterate over the flights association, Hibernate will fetch results in batches of 10. For example if you had an Airport that had 30 flights, if you didn't configure batch fetching you would get 1 query to fetch the Airport and then 30 queries to fetch each flight. With batch fetching you get 1 query to fetch the Airport and 3 queries to fetch each Flight in batches of 10. In other words, batch fetching is an optimization of the lazy fetching strategy. Batch fetching can also be configured at the class level as follows:

class Flight {
    …
    static mapping = {
        batchSize 10
    }
}

Check out part 3 of the GORM Gotchas series for more in-depth coverage of this tricky topic.

5.3.5 Pessimistic and Optimistic Locking

Optimistic Locking

By default GORM classes are configured for optimistic locking. Optimistic locking essentially is a feature of Hibernate which involves storing a version number in a special version column in the database.

The version column gets read into a version property that contains the current versioned state of persistent instance which you can access:

def airport = Airport.get(10)

println airport.version

When you perform updates Hibernate will automatically check the version property against the version column in the database and if they differ will throw a StaleObjectException and the transaction will be rolled back.

This is useful as it allows a certain level of atomicity without resorting to pessimistic locking that has an inherit performance penalty. The downside is that you have to deal with this exception if you have highly concurrent writes. This requires flushing the session:

def airport = Airport.get(10)

try { airport.name = "Heathrow" airport.save(flush:true) } catch (org.springframework.dao.OptimisticLockingFailureException e) { // deal with exception }

The way you deal with the exception depends on the application. You could attempt a programmatic merge of the data or go back to the user and ask them to resolve the conflict.

Alternatively, if it becomes a problem you can resort to pessimistic locking.

The version will only be updated after flushing the session.

Pessimistic Locking

Pessimistic locking is equivalent to doing a SQL "SELECT * FOR UPDATE" statement and locking a row in the database. This has the implication that other read operations will be blocking until the lock is released.

In Grails pessimistic locking is performed on an existing instance via the lock method:

def airport = Airport.get(10)
airport.lock() // lock for update
airport.name = "Heathrow"
airport.save()

Grails will automatically deal with releasing the lock for you once the transaction has been committed. However, in the above case what we are doing is "upgrading" from a regular SELECT to a SELECT..FOR UPDATE and another thread could still have updated the record in between the call to get() and the call to lock().

To get around this problem you can use the static lock method that takes an id just like get:

def airport = Airport.lock(10) // lock for update
airport.name = "Heathrow"
airport.save()

In this case only SELECT..FOR UPDATE is issued.

Though Grails, through Hibernate, supports pessimistic locking, the embedded HSQLDB shipped with Grails which is used as the default in-memory database does not. If you need to test pessimistic locking you will need to do so against a database that does have support such as MySQL.

As well as the lock method you can also obtain a pessimistic locking using queries. For example using a dynamic finder:

def airport = Airport.findByName("Heathrow", [lock:true])

Or using criteria:

def airport = Airport.createCriteria().get {
    eq('name', 'Heathrow')
    lock true
}

5.3.6 Modification Checking

Once you have loaded and possibly modified a persistent domain class instance, it isn't straightforward to retrieve the original values. If you try to reload the instance using get Hibernate will return the current modified instance from its Session cache. Reloading using another query would trigger a flush which could cause problems if your data isn't ready to be flushed yet. So GORM provides some methods to retrieve the original values that Hibernate caches when it loads the instance (which it uses for dirty checking).

isDirty

You can use the isDirty method to check if any field has been modified:

def airport = Airport.get(10)
assert !airport.isDirty()

airport.properties = params if (airport.isDirty()) { // do something based on changed state }

You can also check if individual fields have been modified:

def airport = Airport.get(10)
assert !airport.isDirty()

airport.properties = params if (airport.isDirty('name')) { // do something based on changed name }

getDirtyPropertyNames

You can use the getDirtyPropertyNames method to retrieve the names of modified fields; this may be empty but will not be null:

def airport = Airport.get(10)
assert !airport.isDirty()

airport.properties = params def modifiedFieldNames = airport.getDirtyPropertyNames() for (fieldName in modifiedFieldNames) { // do something based on changed value }

getPersistentValue

You can use the getPersistentValue method to retrieve the value of a modified field:

def airport = Airport.get(10)
assert !airport.isDirty()

airport.properties = params def modifiedFieldNames = airport.getDirtyPropertyNames() for (fieldName in modifiedFieldNames) { def currentValue = airport."$fieldName" def originalValue = airport.getPersistentValue(fieldName) if (currentValue != originalValue) { // do something based on changed value } }

5.4 Querying with GORM

GORM supports a number of powerful ways to query from dynamic finders, to criteria to Hibernate's object oriented query language HQL.

Groovy's ability to manipulate collections via GPath and methods like sort, findAll and so on combined with GORM results in a powerful combination.

However, let's start with the basics.

Listing instances

If you simply need to obtain all the instances of a given class you can use the list method:

def books = Book.list()

The list method supports arguments to perform pagination:

def books = Book.list(offset:10, max:20)

as well as sorting:

def books = Book.list(sort:"title", order:"asc")

Here, the sort argument is the name of the domain class property that you wish to sort on, and the order argument is either asc for ascending or desc for descending.

Retrieval by Database Identifier

The second basic form of retrieval is by database identifier using the get method:

def book = Book.get(23)

You can also obtain a list of instances for a set of identifiers using getAll:

def books = Book.getAll(23, 93, 81)

5.4.1 Dynamic Finders

GORM supports the concept of dynamic finders. A dynamic finder looks like a static method invocation, but the methods themselves don't actually exist in any form at the code level.

Instead, a method is auto-magically generated using code synthesis at runtime, based on the properties of a given class. Take for example the Book class:

class Book {
    String title
    Date releaseDate
    Author author
}
class Author {
    String name
}

The Book class has properties such as title, releaseDate and author. These can be used by the findBy and findAllBy methods in the form of "method expressions":

def book = Book.findByTitle("The Stand")

book = Book.findByTitleLike("Harry Pot%")

book = Book.findByReleaseDateBetween( firstDate, secondDate )

book = Book.findByReleaseDateGreaterThan( someDate )

book = Book.findByTitleLikeOrReleaseDateLessThan( "%Something%", someDate )

Method Expressions

A method expression in GORM is made up of the prefix such as findBy followed by an expression that combines one or more properties. The basic form is:

Book.findBy([Property][Comparator][Boolean Operator])?[Property][Comparator]

The tokens marked with a '?' are optional. Each comparator changes the nature of the query. For example:

def book = Book.findByTitle("The Stand")

book = Book.findByTitleLike("Harry Pot%")

In the above example the first query is equivalent to equality whilst the latter, due to the Like comparator, is equivalent to a SQL like expression.

The possible comparators include:

  • InList - In the list of given values
  • LessThan - less than the given value
  • LessThanEquals - less than or equal a give value
  • GreaterThan - greater than a given value
  • GreaterThanEquals - greater than or equal a given value
  • Like - Equivalent to a SQL like expression
  • Ilike - Similar to a Like, except case insensitive
  • NotEqual - Negates equality
  • Between - Between two values (requires two arguments)
  • IsNotNull - Not a null value (doesn't require an argument)
  • IsNull - Is a null value (doesn't require an argument)

Notice that the last 3 require different numbers of method arguments compared to the rest, as demonstrated in the following example:

def now = new Date()
def lastWeek = now - 7
def book = Book.findByReleaseDateBetween( lastWeek, now )

books = Book.findAllByReleaseDateIsNull() books = Book.findAllByReleaseDateIsNotNull()

Boolean logic (AND/OR)

Method expressions can also use a boolean operator to combine two criteria:

def books =
    Book.findAllByTitleLikeAndReleaseDateGreaterThan("%Java%", new Date()-30)

In this case we're using And in the middle of the query to make sure both conditions are satisfied, but you could equally use Or:

def books =
    Book.findAllByTitleLikeOrReleaseDateGreaterThan("%Java%", new Date()-30)

At the moment, you can only use dynamic finders with a maximum of two criteria, i.e. the method name can only have one boolean operator. If you need to use more, you should consider using either Criteria or the HQL.

Querying Associations

Associations can also be used within queries:

def author = Author.findByName("Stephen King")

def books = author ? Book.findAllByAuthor(author) : []

In this case if the Author instance is not null we use it in a query to obtain all the Book instances for the given Author.

Pagination & Sorting

The same pagination and sorting parameters available on the list method can also be used with dynamic finders by supplying a map as the final parameter:

def books =
    Book.findAllByTitleLike("Harry Pot%", [max:3,
                                           offset:2,
                                           sort:"title",
                                           order:"desc"])

5.4.2 Criteria

Criteria is a type safe, advanced way to query that uses a Groovy builder to construct potentially complex queries. It is a much better alternative to using StringBuffer.

Criteria can be used either via the createCriteria or withCriteria methods. The builder uses Hibernate's Criteria API, the nodes on this builder map the static methods found in the Restrictions class of the Hibernate Criteria API. Example Usage:

def c = Account.createCriteria()
def results = c {
    between("balance", 500, 1000)
    eq("branch", "London")
    or {
        like("holderFirstName", "Fred%")
        like("holderFirstName", "Barney%")
    }
    maxResults(10)
    order("holderLastName", "desc")
}

This criteria will select up to 10 Account objects matching the following criteria:

  • balance is between 500 and 1000
  • branch is 'London'
  • holderFirstName starts with 'Fred' or 'Barney'

The results will be sorted in descending ordery by holderLastName.

Conjunctions and Disjunctions

As demonstrated in the previous example you can group criteria in a logical OR using a or { } block:

or {
    between("balance", 500, 1000)
    eq("branch", "London")
}

This also works with logical AND:

and {
    between("balance", 500, 1000)
    eq("branch", "London")
}

And you can also negate using logical NOT:

not {
    between("balance", 500, 1000)
    eq("branch", "London")
}

All top level conditions are implied to be AND'd together.

Querying Associations

Associations can be queried by having a node that matches the property name. For example say the Account class had many Transaction objects:

class Account {
    …
    static hasMany = [transactions:Transaction]
    …
}

We can query this association by using the property name transaction as a builder node:

def c = Account.createCriteria()
def now = new Date()
def results = c.list {
    transactions {
        between('date',now-10, now)
    }
}

The above code will find all the Account instances that have performed transactions within the last 10 days. You can also nest such association queries within logical blocks:

def c = Account.createCriteria()
def now = new Date()
def results = c.list {
    or {
        between('created',now-10,now)
        transactions {
            between('date',now-10, now)
        }
    }
}

Here we find all accounts that have either performed transactions in the last 10 days OR have been recently created in the last 10 days.

Querying with Projections

Projections may be used to customise the results. To use projections you need to define a "projections" node within the criteria builder tree. There are equivalent methods within the projections node to the methods found in the Hibernate Projections class:

def c = Account.createCriteria()

def numberOfBranches = c.get { projections { countDistinct('branch') } }

Using SQL Restrictions

You can access Hibernate's SQL Restrictions capabilities.

def c = Person.createCriteria()

def peopleWithShortFirstNames = c.list { sqlRestriction "char_length( first_name ) <= 4" }

Note that the parameter there is SQL. The first_name attribute referenced in the example relates to the persistence model, not the object model. The Person class may have a property named firstName which is mapped to a column in the database named first_name.

Also note that the SQL used here is not necessarily portable across databases.

Using Scrollable Results

You can use Hibernate's ScrollableResults feature by calling the scroll method:

def results = crit.scroll {
    maxResults(10)
}
def f = results.first()
def l = results.last()
def n = results.next()
def p = results.previous()

def future = results.scroll(10) def accountNumber = results.getLong('number')

To quote the documentation of Hibernate ScrollableResults:

A result iterator that allows moving around within the results by arbitrary increments. The Query / ScrollableResults pattern is very similar to the JDBC PreparedStatement/ ResultSet pattern and the semantics of methods of this interface are similar to the similarly named methods on ResultSet.

Contrary to JDBC, columns of results are numbered from zero.

Setting properties in the Criteria instance

If a node within the builder tree doesn't match a particular criterion it will attempt to set a property on the Criteria object itself. Thus allowing full access to all the properties in this class. The below example calls setMaxResults and setFirstResult on the Criteria instance:

import org.hibernate.FetchMode as FM
…
def results = c.list {
    maxResults(10)
    firstResult(50)
    fetchMode("aRelationship", FM.JOIN)
}

Querying with Eager Fetching

In the section on Eager and Lazy Fetching we discussed how to declaratively specify fetching to avoid the N+1 SELECT problem. However, this can also be achieved using a criteria query:

def criteria = Task.createCriteria()
def tasks = criteria.list{
    eq "assignee.id", task.assignee.id
    join 'assignee'
    join 'project'
    order 'priority', 'asc'
}

Notice the usage of the join method: it tells the criteria API to use a JOIN to fetch the named associations with the Task instances. It's probably best not to use this for one-to-many associations though, because you will most likely end up with duplicate results. Instead, use the 'select' fetch mode:

import org.hibernate.FetchMode as FM
…
def results = Airport.withCriteria {
    eq "region", "EMEA"
    fetchMode "flights", FM.SELECT
}
Although this approach triggers a second query to get the flights association, you will get reliable results - even with the maxResults option.

fetchMode and join are general settings of the query and can only be specified at the top-level, i.e. you cannot use them inside projections or association constraints.

An important point to bear in mind is that if you include associations in the query constraints, those associations will automatically be eagerly loaded. For example, in this query:

def results = Airport.withCriteria {
    eq "region", "EMEA"
    flights {
        like "number", "BA%"
    }
}
the flights collection would be loaded eagerly via a join even though the fetch mode has not been explicitly set.

Method Reference

If you invoke the builder with no method name such as:

c { … }

The build defaults to listing all the results and hence the above is equivalent to:

c.list { … }

MethodDescription
listThis is the default method. It returns all matching rows.
getReturns a unique result set, i.e. just one row. The criteria has to be formed that way, that it only queries one row. This method is not to be confused with a limit to just the first row.
scrollReturns a scrollable result set.
listDistinctIf subqueries or associations are used, one may end up with the same row multiple times in the result set, this allows listing only distinct entities and is equivalent to DISTINCT_ROOT_ENTITY of the CriteriaSpecification class.
countReturns the number of matching rows.

5.4.3 Hibernate Query Language (HQL)

GORM classes also support Hibernate's query language HQL, a very complete reference for which can be found Chapter 14. HQL: The Hibernate Query Language of the Hibernate documentation.

GORM provides a number of methods that work with HQL including find, findAll and executeQuery. An example of a query can be seen below:

def results =
      Book.findAll("from Book as b where b.title like 'Lord of the%'")

Positional and Named Parameters

In this case the value passed to the query is hard coded, however you can equally use positional parameters:

def results =
      Book.findAll("from Book as b where b.title like ?", ["The Shi%"])

def author = Author.findByName("Stephen King")
def books = Book.findAll("from Book as book where book.author = ?", [author])

Or even named parameters:

def results =
      Book.findAll("from Book as b where b.title like :search or b.author like :search",
                   [search:"The Shi%"])

def author = Author.findByName("Stephen King")
def books = Book.findAll("from Book as book where book.author = :author",
                         [author: author])

Multiline Queries

If you need to separate the query across multiple lines you can use a line continuation character:

def results = Book.findAll("\
from Book as b, \
     Author as a \
where b.author = a and a.surname = ?", ['Smith'])

Groovy multiline strings will NOT work with HQL queries

Pagination and Sorting

You can also perform pagination and sorting whilst using HQL queries. To do so simply specify the pagination options as a map at the end of the method call and include an "ORDER BY" clause in the HQL:

def results =
      Book.findAll("from Book as b where b.title like 'Lord of the%' order by b.title asc",
                   [max:10, offset:20])

5.5 Advanced GORM Features

The following sections cover more advanced usages of GORM including caching, custom mapping and events.

5.5.1 Events and Auto Timestamping

GORM supports the registration of events as methods that get fired when certain events occurs such as deletes, inserts and updates. The following is a list of supported events:
  • beforeInsert - Executed before an object is initially persisted to the database
  • beforeUpdate - Executed before an object is updated
  • beforeDelete - Executed before an object is deleted
  • beforeValidate - Executed before an object is validated
  • afterInsert - Executed after an object is persisted to the database
  • afterUpdate - Executed after an object has been updated
  • afterDelete - Executed after an object has been deleted
  • onLoad - Executed when an object is loaded from the database

To add an event simply register the relevant closure with your domain class.

Do not attempt to flush the session within an event (such as with obj.save(flush:true)). Since events are fired during flushing this will cause a StackOverflowError.

Event types

The beforeInsert event

Fired before an object is saved to the db

class Person {
   Date dateCreated

def beforeInsert() { dateCreated = new Date() } }

The beforeUpdate event

Fired before an existing object is updated

class Person {
   Date dateCreated
   Date lastUpdated

def beforeInsert() { dateCreated = new Date() } def beforeUpdate() { lastUpdated = new Date() } }

The beforeDelete event

Fired before an object is deleted.

class Person {
   String name
   Date dateCreated
   Date lastUpdated

def beforeDelete() { ActivityTrace.withNewSession { new ActivityTrace(eventName:"Person Deleted",data:name).save() } } }

Notice the usage of withNewSession method above. Since events are triggered whilst Hibernate is flushing using persistence methods like save() and delete() won't result in objects being saved unless you run your operations with a new Session.

Fortunately the withNewSession method allows you to share the same transactional JDBC connection even though you're using a different underlying Session.

The beforeValidate event

Fired before an object is validated.

class Person {
   String name

static constraints = { name size: 5..45 }

def beforeValidate() { name = name?.trim() } }

The beforeValidate method is run before any validators are run.

GORM supports an overloaded version of beforeValidate which accepts a List parameter which may include the names of the properties which are about to be validated. This version of beforeValidate will be called when the validate method has been invoked and passed a List of property names as an argument.

class Person {
   String name
   String town
   Integer age

static constraints = { name size: 5..45 age range: 4..99 }

def beforeValidate(List propertiesBeingValidated) { // do pre validation work based on propertiesBeingValidated } }

def p = new Person(name: 'Jacob Brown', age: 10) p.validate(['age', 'name'])

Note that when validate is triggered indirectly because of a call to the save method that the validate method is being invoked with no arguments, not a List that includes all of the property names.

Either or both versions of beforeValidate may be defined in a domain class. GORM will prefer the List version if a List is passed to validate but will fall back on the no-arg version if the List version does not exist. Likewise, GORM will prefer the no-arg version if no arguments are passed to validate but will fall back on the List version if the no-arg version does not exist. In that case, null is passed to beforeValidate.

The onLoad/beforeLoad event

Fired immediately before an object is loaded from the db:

class Person {
   String name
   Date dateCreated
   Date lastUpdated

def onLoad() { log.debug "Loading ${id}" } }

beforeLoad() is effectively a synonym for onLoad(), so only declare one or the other.

The afterLoad event

Fired immediately after an object is loaded from the db:

class Person {
   String name
   Date dateCreated
   Date lastUpdated

def afterLoad() { name = "I'm loaded" } }

Custom Event Listeners

You can also register event handler classes in an application's grails-app/conf/spring/resources.groovy or in the doWithSpring closure in a plugin descriptor by registering a Spring bean named hibernateEventListeners. This bean has one property, listenerMap which specifies the listeners to register for various Hibernate events.

The values of the map are instances of classes that implement one or more Hibernate listener interfaces. You can use one class that implements all of the required interfaces, or one concrete class per interface, or any combination. The valid map keys and corresponding interfaces are listed here:

NameInterface
auto-flushAutoFlushEventListener
mergeMergeEventListener
createPersistEventListener
create-onflushPersistEventListener
deleteDeleteEventListener
dirty-checkDirtyCheckEventListener
evictEvictEventListener
flushFlushEventListener
flush-entityFlushEntityEventListener
loadLoadEventListener
load-collectionInitializeCollectionEventListener
lockLockEventListener
refreshRefreshEventListener
replicateReplicateEventListener
save-updateSaveOrUpdateEventListener
saveSaveOrUpdateEventListener
updateSaveOrUpdateEventListener
pre-loadPreLoadEventListener
pre-updatePreUpdateEventListener
pre-deletePreDeleteEventListener
pre-insertPreInsertEventListener
pre-collection-recreatePreCollectionRecreateEventListener
pre-collection-removePreCollectionRemoveEventListener
pre-collection-updatePreCollectionUpdateEventListener
post-loadPostLoadEventListener
post-updatePostUpdateEventListener
post-deletePostDeleteEventListener
post-insertPostInsertEventListener
post-commit-updatePostUpdateEventListener
post-commit-deletePostDeleteEventListener
post-commit-insertPostInsertEventListener
post-collection-recreatePostCollectionRecreateEventListener
post-collection-removePostCollectionRemoveEventListener
post-collection-updatePostCollectionUpdateEventListener

For example, you could register a class AuditEventListener which implements PostInsertEventListener, PostUpdateEventListener, and PostDeleteEventListener using the following in an application:

beans = {

auditListener(AuditEventListener)

hibernateEventListeners(HibernateEventListeners) { listenerMap = ['post-insert':auditListener, 'post-update':auditListener, 'post-delete':auditListener] } }

or use this in a plugin:

def doWithSpring = {

auditListener(AuditEventListener)

hibernateEventListeners(HibernateEventListeners) { listenerMap = ['post-insert':auditListener, 'post-update':auditListener, 'post-delete':auditListener] } }

Automatic timestamping

The examples above demonstrated using events to update a lastUpdated and dateCreated property to keep track of updates to objects. However, this is actually not necessary. By merely defining a lastUpdated and dateCreated property these will be automatically updated for you by GORM.

If this is not the behaviour you want you can disable this feature with:

class Person {
   Date dateCreated
   Date lastUpdated
   static mapping = {
      autoTimestamp false
   }
}

If you put nullable: false constraints on either dateCreated or lastUpdated, your domain instances will fail validation - probably not what you want. Leave constraints off these properties unless you have disabled automatic timestamping.

5.5.2 Custom ORM Mapping

Grails domain classes can be mapped onto many legacy schemas via an Object Relational Mapping Domain Specify Language. The following sections takes you through what is possible with the ORM DSL.

None if this is necessary if you are happy to stick to the conventions defined by GORM for table, column names and so on. You only needs this functionality if you need to in anyway tailor the way GORM maps onto legacy schemas or performs caching

Custom mappings are defined using a a static mapping block defined within your domain class:

class Person {
  ..
  static mapping = {

} }

5.5.2.1 Table and Column Names

Table names

The database table name which the class maps to can be customized using a call to table:

class Person {
    ..
    static mapping = {
        table 'people'
    }
}

In this case the class would be mapped to a table called people instead of the default name of person.

Column names

It is also possible to customize the mapping for individual columns onto the database. For example if its the name you want to change you can do:

class Person {
    String firstName
    static mapping = {
        table 'people'
        firstName column:'First_Name'
    }
}

In this case we define method calls that match each property name (in this case firstName). We then use the named parameter column, to specify the column name to map onto.

Column type

GORM supports configuration of Hibernate types via the DSL using the type attribute. This includes specifing user types that subclass the Hibernate org.hibernate.usertype.UserType class, which allows complete customization of how a type is persisted. As an example if you had a PostCodeType you could use it as follows:

class Address {
    String number
    String postCode
    static mapping = {
        postCode type:PostCodeType
    }
}

Alternatively if you just wanted to map it to one of Hibernate's basic types other than the default chosen by Grails you could use:

class Address {
    String number
    String postCode
    static mapping = {
        postCode type:'text'
    }
}

This would make the postCode column map to the SQL TEXT or CLOB type depending on which database is being used.

See the Hibernate documentation regarding Basic Types for further information.

Many-to-One/One-to-One Mappings

In the case of associations it is also possible to change the foreign keys used to map associations. In the case of a many- or one-to-one association this is exactly the same as any regular column. For example consider the following:

class Person {
    String firstName
    Address address

static mapping = { table 'people' firstName column:'First_Name' address column:'Person_Address_Id' } }

By default the address association would map to a foreign key column called address_id. By using the above mapping we have changed the name of the foreign key column to Person_Adress_Id.

One-to-Many Mapping

With a bidirectional one-to-many you can change the foreign key column used simple by changing the column name on the many side of the association as per the example in the previous section on one-to-one associations. However, with unidirectional association the foreign key needs to be specified on the association itself. For example given a unidirectional one-to-many relationship between Person and Address the following code will change the foreign key in the address table:

class Person {
    String firstName
    static hasMany = [addresses:Address]
    static mapping = {
        table 'people'
        firstName column:'First_Name'
        addresses column:'Person_Address_Id'
    }
}

If you don't want the column to be in the address table, but instead some intermediate join table you can use the joinTable parameter:

class Person {
    String firstName
    static hasMany = [addresses:Address]
    static mapping = {
        table 'people'
        firstName column:'First_Name'
        addresses joinTable:[name:'Person_Addresses', key:'Person_Id', column:'Address_Id']
    }
}

Many-to-Many Mapping

Grails, by default maps a many-to-many association using a join table. For example consider the below many-to-many association:

class Group {
    …
    static hasMany = [people:Person]
}
class Person {
    …
    static belongsTo = Group
    static hasMany = [groups:Group]
}

In this case Grails will create a join table called group_person containing foreign keys called person_id and group_id referencing the person and group tables. If you need to change the column names you can specify a column within the mappings for each class.

class Group {
   …
   static mapping = {
       people column:'Group_Person_Id'
   }
}
class Person {
   …
   static mapping = {
       groups column:'Group_Group_Id'
   }
}

You can also specify the name of the join table to use:

class Group {
   …
   static mapping = {
       people column:'Group_Person_Id',joinTable:'PERSON_GROUP_ASSOCIATIONS'
   }
}
class Person {
   …
   static mapping = {
       groups column:'Group_Group_Id',joinTable:'PERSON_GROUP_ASSOCIATIONS'
   }
}

5.5.2.2 Caching Strategy

Setting up caching

Hibernate features a second-level cache with a customizable cache provider. This needs to be configured in the grails-app/conf/DataSource.groovy file as follows:

hibernate {
    cache.use_second_level_cache=true
    cache.use_query_cache=true
    cache.provider_class='org.hibernate.cache.EhCacheProvider'
}

You can of course customize these settings how you desire, for example if you want to use a distributed caching mechanism.

For further reading on caching and in particular Hibernate's second-level cache, refer to the Hibernate documentation on the subject.

Caching instances

In your mapping block to enable caching with the default settings use a call to the cache method:

class Person {
  ..
  static mapping = {
      table 'people'
      cache true
  }
}

This will configure a 'read-write' cache that includes both lazy and non-lazy properties. If you need to customize this further you can do:

class Person {
  ..
  static mapping = {
      table 'people'
      cache usage:'read-only', include:'non-lazy'
  }
}

Caching associations

As well as the ability to use Hibernate's second level cache to cache instances you can also cache collections (associations) of objects. For example:

class Person {
  String firstName
  static hasMany = [addresses:Address]
  static mapping = {
      table 'people'
      version false
      addresses column:'Address', cache:true
  }
}
class Address {
   String number
   String postCode
}

This will enable a 'read-write' caching mechanism on the addresses collection. You can also use:

cache:'read-write' // or 'read-only' or 'transactional'

To further configure the cache usage.

Caching Queries

You can cache queries such as dynamic finders and criteria. To do so using a dynamic finder you can pass the cache argument:

def person = Person.findByFirstName("Fred", [cache:true])

Note that in order for the results of the query to be cached, you still need to enable caching in your mapping as discussed in the previous section.

You can also cache criteria queries:

def people = Person.withCriteria {
    like('firstName', 'Fr%')
    cache true
}

Cache usages

Below is a description of the different cache settings and their usages:

  • read-only - If your application needs to read but never modify instances of a persistent class, a read-only cache may be used.
  • read-write - If the application needs to update data, a read-write cache might be appropriate.
  • nonstrict-read-write - If the application only occasionally needs to update data (ie. if it is extremely unlikely that two transactions would try to update the same item simultaneously) and strict transaction isolation is not required, a nonstrict-read-write cache might be appropriate.
  • transactional - The transactional cache strategy provides support for fully transactional cache providers such as JBoss TreeCache. Such a cache may only be used in a JTA environment and you must specify hibernate.transaction.manager_lookup_class in the grails-app/conf/DataSource.groovy file's hibernate config.

5.5.2.3 Inheritance Strategies

By default GORM classes uses table-per-hierarchy inheritance mapping. This has the disadvantage that columns cannot have a NOT-NULL constraint applied to them at the db level. If you would prefer to use a table-per-subclass inheritance strategy you can do so as follows:

class Payment {
    Long id
    Long version
    Integer amount

static mapping = { tablePerHierarchy false } } class CreditCardPayment extends Payment { String cardNumber }

The mapping of the root Payment class specifies that it will not be using table-per-hierarchy mapping for all child classes.

5.5.2.4 Custom Database Identity

You can customize how GORM generates identifiers for the database using the DSL. By default GORM relies on the native database mechanism for generating ids. This is by far the best approach, but there are still many schemas that have different approaches to identity.

To deal with this Hibernate defines the concept of an id generator. You can customize the id generator and the column it maps to as follows:

class Person {
  ..
  static mapping = {
      table 'people'
      version false
      id generator:'hilo', params:[table:'hi_value',column:'next_value',max_lo:100]
  }
}

In this case we're using one of Hibernate's built in 'hilo' generators that uses a separate table to generate ids.

For more information on the different Hibernate generators refer to the Hibernate reference documentation

Note that if you want to merely customise the column that the id lives on you can do:

class Person {
  ..
  static mapping = {
      table 'people'
      version false
      id column:'person_id'
  }
}

5.5.2.5 Composite Primary Keys

GORM supports the concept of composite identifiers (identifiers composed from 2 or more properties). It is not an approach we recommend, but is available to you if you need it:

class Person {
  String firstName
  String lastName

static mapping = { id composite:['firstName', 'lastName'] } }

The above will create a composite id of the firstName and lastName properties of the Person class. When you later need to retrieve an instance by id you have to use a prototype of the object itself:

def p = Person.get(new Person(firstName:"Fred", lastName:"Flintstone"))
println p.firstName

5.5.2.6 Database Indices

To get the best performance out of your queries it is often necessary to tailor the table index definitions. How you tailor them is domain specific and a matter of monitoring usage patterns of your queries. With GORM's DSL you can specify which columns need to live in which indexes:

class Person {
  String firstName
  String address
  static mapping = {
      table 'people'
      version false
      id column:'person_id'
      firstName column:'First_Name', index:'Name_Idx'
      address column:'Address', index:'Name_Idx,Address_Index'
  }
}

Note that you cannot have any spaces in the value of the index attribute; in this example index:'Name_Idx, Address_Index' will cause an error.

5.5.2.7 Optimistic Locking and Versioning

As discussed in the section on Optimistic and Pessimistic Locking, by default GORM uses optimistic locking and automatically injects a version property into every class which is in turn mapped to a version column at the database level.

If you're mapping to a legacy schema this can be problematic, so you can disable this feature by doing the following:

class Person {
    ..
    static mapping = {
        table 'people'
        version false
    }
}

If you disable optimistic locking you are essentially on your own with regards to concurrent updates and are open to the risk of users losing (due to data overriding) data unless you use pessimistic locking

5.5.2.8 Eager and Lazy Fetching

Lazy Collections

As discussed in the section on Eager and Lazy fetching, GORM collections are lazily loaded by default but you can change this behaviour via the ORM DSL. There are several options available to you, but the most common ones are:

  • lazy: false
  • fetch: 'join'

and they're used like this:

class Person {
    String firstName
    Pet pet
    static hasMany = [addresses:Address]
    static mapping = {
        addresses lazy:false
        pet fetch: 'join'
    }
}

class Address { String street String postCode }

class Pet { String name }

The first option, lazy: false , ensures that when a Person instance is loaded, its addresses collection is loaded at the same time with a second SELECT. The second option is basically the same, except the collection is loaded via a JOIN rather than another SELECT. Typically you will want to reduce the number of queries, so fetch: 'join' will be the more appropriate option. On the other hand, it could feasibly be the more expensive approach if your domain model and data result in more and larger results than would otherwise be necessary.

For more advanced users, the other settings available are:

  1. batchSize: N
  2. lazy: false, batchSize: N

where N is an integer. These allow you to fetch results in batches, with one query per batch. As a simple example, consider this mapping for Person:

class Person {
    String firstName
    Pet pet

static mapping = { pet batchSize: 5 } }

If a query returns multiple Person instances, then when we access the first pet property, Hibernate will fetch that Pet plus the four next ones. You can get the same behaviour with eager loading by combining batchSize with the lazy: false option. You can find out more about these options in the Hibernate user guide and this primer on fetching strategies. Note that ORM DSL does not currently support the "subselect" fetching strategy.

Lazy Single-Ended Associations

In GORM, one-to-one and many-to-one associations are by default lazy. Non-lazy single ended associations can be problematic when you load many entities because each non-lazy association will result in an extra SELECT statement. If the associated entities also have non-lazy associations, the number of queries grows exponentially!

If you want to make a one-to-one or many-to-one association non-lazy/eager, you can use the same technique as for lazy collections:

class Person {
    String firstName
}

class Address { String street String postCode static belongsTo = [person:Person] static mapping = { person lazy:false } }

Here we configure GORM to load the associated Person instance (through the person property) whenever an Address is loaded.

Lazy Single-Ended Associations and Proxies

In order to facilitate single-ended lazy associations Hibernate uses runtime generated proxies. The way this works is that Hibernate dynamically subclasses the proxied entity to create the proxy.

Consider the previous example but with a lazily-loaded person association: Hibernate will set the person property to a proxy that is a subclass of Person. When you call any of the getters or setters on that proxy, Hibernate will load the entity from the database.

Unfortunately this technique can produce surprising results. Consider the following example classes:

class Pet {
    String name
}
class Dog extends Pet {
}
class Person {
    String name
    Pet pet
}

and assume that we have a single Person instance with a Dog as his or her pet. The following code will work as you would expect:

def person = Person.get(1)
assert person.pet instanceof Dog
assert Pet.get(person.petId) instanceof Dog
But this won't:
def person = Person.get(1)
assert person.pet instanceof Dog
assert Pet.list()[0] instanceof Dog
For some reason, the second assertion fails. To add to the confusion, this will work:
assert Pet.list()[0] instanceof Dog
What's going on here? It's down to a combination of how proxies work and the guarantees that the Hibernate session makes. When you load the Person instance, Hibernate creates a proxy for its pet relation and attaches it to the session. Once that happens, whenever you retrieve that Pet instance via a query, a get(), or the pet relation within the same session , Hibernate gives you the proxy.

Fortunately for us, GORM automatically unwraps the proxy when you use get() and findBy*(), or when you directly access the relation. That means you don't have to worry at all about proxies in the majority of cases. But GORM doesn't do that for objects returned via a query that returns a list, such as list() and findAllBy*(). However, if Hibernate hasn't attached the proxy to the session, those queries will return the real instances - hence why the last example works.

You can protect yourself to a degree from this problem by using the instanceOf method by GORM:

def person = Person.get(1)
assert Pet.list()[0].instanceOf(Dog)

However, it won't help here casting is involved. For example, the following code will throw a ClassCastException because the first pet in the list is a proxy instance with a class that is neither Dog nor a sub-class of Dog:

def person = Person.get(1)
Dog pet = Pet.list()[0]

Of course, it's best not to use static types in this situation. If you use an untyped variable for the pet instead, you can access any Dog properties or methods on the instance without any problems.

These days it's rare that you will come across this issue, but it's best to be aware of it just in case. At least you will know why such an error occurs and be able to work around it.

5.5.2.9 Custom Cascade Behaviour

As describes in the section on cascading updates, the primary mechanism to control the way updates and deletes are cascading from one association to another is the belongsTo static property.

However, the ORM DSL gives you complete access to Hibernate's transitive persistence capabilities via the cascade attribute.

Valid settings for the cascade attribute include:

  • merge - merges the state of a detached association
  • save-update - cascades only saves and updates to an association
  • delete - cascades only deletes to an association
  • lock - useful if a pessimistic lock should be cascaded to its associations
  • refresh - cascades refreshes to an association
  • evict - cascades evictions (equivalent to discard() in GORM) to associations if set
  • all - cascade ALL operations to associations
  • all-delete-orphan - Applies only to one-to-many associations and indicates that when a child is removed from an association then it should be automatically deleted. Children are also deleted when the parent is.

It is advisable to read the section in the Hibernate documentation on transitive persistence to obtain a better understanding of the different cascade styles and recommendation for their usage

To specific the cascade attribute simply define one or many (comma-separated) of the aforementioned settings as its value:

class Person {
  String firstName
  static hasMany = [addresses:Address]
  static mapping = {
      addresses cascade:"all-delete-orphan"
  }
}
class Address {
  String street
  String postCode
}

5.5.2.10 Custom Hibernate Types

You saw in an earlier section that you can use composition (via the embedded property) to break a table into multiple objects. You can achieve a similar effect via Hibernate's custom user types. These are not domain classes themselves, but plain Java or Groovy classes with associated. Each of these types also has a corresponding "meta-type" class that implements org.hibernate.usertype.UserType.

The Hibernate reference manual has some information on custom types, but here we will focus on how to map them in Grails. Let's start by taking a look at a simple domain class that uses an old-fashioned (pre-Java 1.5) type-safe enum class:

class Book {
  String title
  String author
  Rating rating

static mapping = { rating type: RatingUserType } }

All we have done is declare the rating field the enum type and set the property's type in the custom mapping to the corresponding UserType implementation. That's all you have to do to start using your custom type. If you want, you can also use the other column settings such as "column" to change the column name and "index" to add it to an index.

Custom types aren't limited to just a single column - they can be mapped to as many columns as you want. In such cases you need to explicitly define in the mapping what columns to use, since Hibernate can only use the property name for a single column. Fortunately, Grails allows you to map multiple columns to a property using this syntax:

class Book {
  String title
  Name author
  Rating rating

static mapping = { name type: NameUserType, { column name: "first_name" column name: "last_name" } rating type: RatingUserType } }

The above example will create "first_name" and "last_name" columns for the author property. You'll be pleased to know that you can also use some of the normal column/property mapping attributes in the column definitions. For example:

column name: "first_name", index: "my_idx", unique: true

The column definitions do not support the following attributes: type, cascade, lazy, cache, and joinTable.

One thing to bear in mind with custom types is that they define the SQL types for the corresponding database columns. That helps take the burden of configuring them yourself, but what happens if you have a legacy database that uses a different SQL type for one of the columns? In that case, you need to override column's SQL type using the sqlType attribute:

class Book {
  String title
  Name author
  Rating rating

static mapping = { name type: NameUserType, { column name: "first_name", sqlType: "text" column name: "last_name", sqlType: "text" } rating type: RatingUserType, sqlType: "text" } }

Mind you, the SQL type you specify needs to still work with the custom type. So overriding a default of "varchar" with "text" is fine, but overriding "text" with "yes_no" isn't going to work.

5.5.2.11 Derived Properties

A derived property is a property that takes its value from a SQL expression, often but not necessarily based on the value of some other persistent property. Consider a Product class like this:

class Product {
    Float price
    Float taxRate
    Float tax
}

If the tax property is derived based on the value of price and taxRate properties then there may be no need to persist the tax property in the database. The SQL used to derive the value of a derived property may be expressed in the ORM DSL like this:

class Product {
    Float price
    Float taxRate
    Float tax

static mapping = { tax formula: 'PRICE * TAX_RATE' } }

Note that the formula expressed in the ORM DSL is SQL so references to other properties should relate to the persistence model not the object model, which is why the example refers to PRICE and TAX_RATE instead of price and taxRate.

With that in place, when a Product is retrieved with something like Product.get(42), the SQL that is generated to support that will look something like this:

select
    product0_.id as id1_0_,
    product0_.version as version1_0_,
    product0_.price as price1_0_,
    product0_.tax_rate as tax4_1_0_,
    product0_.PRICE * product0_.TAX_RATE as formula1_0_
from
    product product0_
where
    product0_.id=?

Since the tax property is being derived at runtime and not stored in the database it might seem that the same effect could be achieved by adding a method like getTax() to the Product class that simply returns the product of the taxRate and price properties. With an approach like that you would give up the ability query the database based on the value of the tax property. Using a derived property allows exactly that. To retrieve all Product objects that have a tax value greater than 21.12 you could execute a query like this:

Product.findAllByTaxGreaterThan(21.12)

Derived properties may be referenced in the Criteria API:

Product.withCriteria {
    gt 'tax', 21.12f
}

The SQL that is generated to support either of those would look something like this:

select
    this_.id as id1_0_,
    this_.version as version1_0_,
    this_.price as price1_0_,
    this_.tax_rate as tax4_1_0_,
    this_.PRICE * this_.TAX_RATE as formula1_0_
from
    product this_
where
    this_.PRICE * this_.TAX_RATE>?

Note that because the value of a derived property is generated in the database and depends on the execution of SQL code, derived properties may not have GORM constraints applied to them. If constraints are specified for a derived property, they will be ignored.

5.5.2.12 Custom Naming Strategy

By default Grails uses Hibernate's ImprovedNamingStrategy to convert domain class Class and field names to SQL table and column names by converting from camel-cased Strings to ones that use underscores as word separators. You can customize these on a per-instance basis in the mapping closure but if there's a consistent pattern you can specify a different NamingStrategy class to use.

Configure the class name to be used in grails-app/conf/DataSource.groovy in the hibernate section, e.g.

dataSource {
    pooled = true
    dbCreate = "create-drop"
    …
}

hibernate { cache.use_second_level_cache = true … naming_strategy = com.myco.myproj.CustomNamingStrategy }

You can use an existing class or write your own, for example one that prefixes table names and column names:

package com.myco.myproj

import org.hibernate.cfg.ImprovedNamingStrategy import org.hibernate.util.StringHelper

class CustomNamingStrategy extends ImprovedNamingStrategy {

String classToTableName(String className) { "table_" + StringHelper.unqualify(className) }

String propertyToColumnName(String propertyName) { "col_" + StringHelper.unqualify(propertyName) } }

5.5.3 Default Sort Order

You can sort objects using queries arguments such as those found in the list method:

def airports = Airport.list(sort:'name')

However, you can also declare the default sort order for a collection in the mapping:

class Airport {
    …
    static mapping = {
        sort "name"
    }
}

The above means that all collections of Airports will be sorted by default by the airport name. If you also want to change the sort order , use this syntax:

class Airport {
    …
    static mapping = {
        sort name:"desc"
    }
}

Finally, you can configure the sorting at the association level:

class Airport {
    …
    static hasMany = [flights:Flight]

static mapping = { flights sort:'number', order:'desc' } }

In this case, the flights collection will always be sorted in descending order of flight number.

These mappings will not work for default unidirectional one-to-many or many-to-many relationships because they involve a join table. See this issue for more details. Consider using a SortedSet or queries with sort parameters to fetch the data you need.

5.6 Programmatic Transactions

Grails is built on Spring and hence uses Spring's Transaction abstraction for dealing with programmatic transactions. However, GORM classes have been enhanced to make this more trivial through the withTransaction method which accepts a block the first argument to which is the Spring TransactionStatus object.

A typical usage scenario is as follows:

def transferFunds = {
    Account.withTransaction { status ->
        def source = Account.get(params.from)
        def dest = Account.get(params.to)

def amount = params.amount.toInteger() if (source.active) { source.balance -= amount if (dest.active) { dest.amount += amount } else { status.setRollbackOnly() } } } }

In this example we rollback the transactions if the destination account is not active and if any exception are thrown during the process the transaction will automatically be rolled back.

You can also use "save points" to rollback a transaction to a particular point in time if you don't want to rollback the entire transaction. This can be achieved through the use of Spring's SavePointManager interface.

The withTransaction method deals with the begin/commit/rollback logic for you within the scope of the block.

5.7 GORM and Constraints

Although constraints are covered in the Validation section, it is important to mention them here as some of the constraints can affect the way in which the database schema is generated.

Where feasible, Grails uses a domain class's constraints to influence the database columns generated for the corresponding domain class properties.

Consider the following example. Suppose we have a domain model with the following property.

String name
String description

By default, in MySQL, Grails would define these columns as...

column name | data type
description | varchar(255)

But perhaps the business rules for this domain class state that a description can be up to 1000 characters in length. If that were the case, we'd likely define the column as follows if we were creating the table via an SQL script.

column name | data type
description | TEXT

Chances are we'd also want to have some application-based validation to make sure we don't exceed that 1000 character limit before we persist any records. In Grails, we achieve this validation via constraints. We'd add the following constraint declaration to the domain class.

static constraints = {
    description(maxSize:1000)
}

This constraint would provide both the application-based validation we want and it would also cause the schema to be generated as shown above. Below is a description of the other constraints that influence schema generation.

Constraints Affecting String Properties

If either the maxSize or the size constraint is defined, Grails sets the maximum column length based on the constraint value.

In general, it's not advisable to use both constraints on the same domain class property. However, if both the maxSize constraint and the size constraint are defined, then Grails sets the column length to the minimum of the maxSize constraint and the upper bound of the size constraint. (Grails uses the minimum of the two, because any length that exceeds that minimum will result in a validation error.)

If the inList constraint is defined (and the maxSize and the size constraints are not defined), then Grails sets the maximum column length based on the length of the longest string in the list of valid values. For example, given a list including values "Java", "Groovy", and "C++", Grails would set the column length to 6 (i.e., the number of characters in the string "Groovy").

Constraints Affecting Numeric Properties

If the max constraint, the min constraint, or the range constraint is defined, Grails attempts to set the column precision based on the constraint value. (The success of this attempted influence is largely dependent on how Hibernate interacts with the underlying DBMS.)

In general, it's not advisable to combine the pair min/max and range constraints together on the same domain class property. However, if both of these constraints is defined, then Grails uses the minimum precision value from the constraints. (Grails uses the minimum of the two, because any length that exceeds that minimum precision will result in a validation error.)

If the scale constraint is defined, then Grails attempts to set the column scale based on the constraint value. This rule only applies to floating point numbers (i.e., java.lang.Float, java.Lang.Double, java.lang.BigDecimal, or subclasses of java.lang.BigDecimal). (The success of this attempted influence is largely dependent on how Hibernate interacts with the underlying DBMS.)

The constraints define the minimum/maximum numeric values, and Grails derives the maximum number of digits for use in the precision. Keep in mind that specifying only one of min/max constraints will not affect schema generation (since there could be large negative value of property with max:100, for example), unless specified constraint value requires more digits than default Hibernate column precision is (19 at the moment). For example...

someFloatValue(max:1000000, scale:3)

would yield:

someFloatValue DECIMAL(19, 3) // precision is default

but

someFloatValue(max:12345678901234567890, scale:5)

would yield:

someFloatValue DECIMAL(25, 5) // precision = digits in max + scale

and

someFloatValue(max:100, min:-100000)

would yield:

someFloatValue DECIMAL(8, 2) // precision = digits in min + default scale