Grails 2.0

<select:select
   optionKey="id"
   from="${UserGroup.list()}"
   name="userGroup.id"
   value="${userGroupPermission?.userGroup?.id}"
   optionValue="name"                                
   valueMessagePrefix="label.constant.defaultUserGroup"/>

import org.springframework.beans.SimpleTypeConverter
import org.springframework.web.servlet.support.RequestContextUtils as RCU
import org.codehaus.groovy.grails.commons.DomainClassArtefactHandler
class SelectTagLib {
    static namespace = 'select'
    def out // to facilitate testing
    /**
     * A helper tag for creating HTML selects
     *
     * Examples:
     * <g:select name="user.age" from="${18..65}" value="${age}" />
     * <g:select name="user.company.id" from="${Company.list()}" value="${user?.company.id}" optionKey="id" />
     */
    def select = {attrs ->
        def messageSource = grailsAttributes.applicationContext.getBean('messageSource')
        def locale = RCU.getLocale(request)
        def writer = out
        attrs.id = attrs.id ? attrs.id : attrs.name
        def from = attrs.remove('from')
        def keys = attrs.remove('keys')
        def optionKey = attrs.remove('optionKey')
        def optionValue = attrs.remove('optionValue')
        def value = attrs.remove('value')
        if (value instanceof Collection && attrs.multiple == null) {
            attrs.multiple = 'multiple'
        }
        def valueMessagePrefix = attrs.remove('valueMessagePrefix')
        def noSelection = attrs.remove('noSelection')
        if (noSelection != null) {
            noSelection = noSelection.entrySet().iterator().next()
        }
        def disabled = attrs.remove('disabled')
        if (disabled && Boolean.valueOf(disabled)) {
            attrs.disabled = 'disabled'
        }
        writer << "<select name="${attrs.remove('name')}" "
        // process remaining attributes
        outputAttributes(attrs)
        writer << '>'
        writer.println()
        if (noSelection) {
            renderNoSelectionOption(noSelection.key, noSelection.value, value)
            writer.println()
        }
        // create options from list
        if (from) {
            from.eachWithIndex {el, i ->
                def keyValue = null
                writer << '<option '
                if (keys) {
                    keyValue = keys[i]
                    writeValueAndCheckIfSelected(keyValue, value, writer)
                }
                else if (optionKey) {
                    if (optionKey instanceof Closure) {
                        keyValue = optionKey(el)
                    }
                    else if (el != null && optionKey == 'id' && grailsApplication.getArtefact(DomainClassArtefactHandler.TYPE, el.getClass().name)) {
                        keyValue = el.ident()
                    }
                    else {
                        keyValue = el[optionKey]
                    }
                    writeValueAndCheckIfSelected(keyValue, value, writer)
                }
                else {
                    keyValue = el
                    writeValueAndCheckIfSelected(keyValue, value, writer)
                }
                writer << '>'
                if (valueMessagePrefix) {
                    def optionValueStr = optionValueToString(el, optionValue)
                    def messageArgs = optionValue ? optionValueStr : keyValue
                    def message = messageSource.getMessage("${valueMessagePrefix}.${messageArgs}", null, null, locale)
                    if (message != null) {
                        writer << message.encodeAsHTML()
                    }
                    else if (optionValue) {
                        writer << optionValueStr
                    }
                    else if (keyValue) {
                            writer << keyValue.encodeAsHTML()
                        }
                        else {
                            def s = el.toString()
                            if (s) writer << s.encodeAsHTML()
                        }
                }
                else if (optionValue) {
                    writer << optionValueToString(el, optionValue)
                }
                else {
                    def s = el.toString()
                    if (s) writer << s.encodeAsHTML()
                }
                writer << '</option>'
                writer.println()
            }
        }
        // close tag
        writer << '</select>'
    }

    def typeConverter = new SimpleTypeConverter()
    private writeValueAndCheckIfSelected(keyValue, value, writer) {
        boolean selected = false
        def keyClass = keyValue?.getClass()
        if (keyClass.isInstance(value)) {
            selected = (keyValue == value)
        }
        else if (value instanceof Collection) {
            selected = value.contains(keyValue)
        }
        else if (keyClass && value) {
                try {
                    value = typeConverter.convertIfNecessary(value, keyClass)
                    selected = (keyValue == value)
                } catch (Exception) {
                    // ignore
                }
            }
        writer << "value="${keyValue}" "
        if (selected) {
            writer << 'selected="selected" '
        }
    }

    /**
     * Dump out attributes in HTML compliant fashion
     */
    void outputAttributes(attrs) {
        attrs.remove('tagName') // Just in case one is left
        attrs.each {k, v ->
            out << k << "="" << v.encodeAsHTML() << "" "
        }
    }
    def renderNoSelectionOption = {noSelectionKey, noSelectionValue, value ->
        // If a label for the '--Please choose--' first item is supplied, write it out
        out << '<option value="' << (noSelectionKey == null ? "" : noSelectionKey) << '"'
        if (noSelectionKey.equals(value)) {
            out << ' selected="selected" '
        }
        out << '>' << noSelectionValue.encodeAsHTML() << '</option>'
    }
    private String optionValueToString(def el, def optionValue) {
        if (optionValue instanceof Closure) {
            return optionValue(el).toString().encodeAsHTML()
        }
        el[optionValue].toString().encodeAsHTML()
    }
}