Grails bind request parameters to enum

Question

My Grails application has a large number of enums that look like this:

public enum Rating {
    BEST("be"), GOOD("go"), AVERAGE("av"), BAD("ba"), WORST("wo")
    final String id

    private RateType(String id) {
        this.id = id
    }

    static public RateType getEnumFromId(String value) {
        values().find {it.id == value }
    }   
}

If I have a command object such as this:

class MyCommand {
    Rating rating
}

I would like to (for example) automatically convert a request parameter with value "wo" to Rating.WORST.

The procedure for defining custom converters is described here (in the context of converting Strings to Dates). Although this procedure works fine, I don't want to have to create a class implementing PropertyEditorSupport for each of my enums. Is there a better alternative?

Answer 1

I found a solution I'm pretty happy with.

Step 1: Create an implementation of PropertyEditorSupport to convert text to/from the relevant Enum

public class EnumEditor extends PropertyEditorSupport {

    private Class<? extends Enum<?>> clazz

    public EnumEditor(Class<? extends Enum<?>> clazz) {
        this.clazz = clazz
    }

    public String getAsText() {
        return value?.id
    }

    public void setAsText(String text) {
        value = clazz.getEnumFromId(text)
    }
}

Step 2: Define a class that registers EnumEditor as a converter for the various enum classes. To change the list of enum classes that are bindable by id, just modify BINDABLE_ENUMS

public class CustomPropertyEditorRegistrar implements PropertyEditorRegistrar {

    private static final String REQUIRED_METHOD_NAME = 'getEnumFromId'

    // Add any enums that you want to bind to by ID into this list
    private static final BINDABLE_ENUMS = [Rating, SomeOtherEnum, SomeOtherEnum2]

    public void registerCustomEditors(PropertyEditorRegistry registry) {            

        BINDABLE_ENUMS.each {enumClass ->
            registerEnum(registry, enumClass)
        }
    }

    /**
     * Register an enum to be bound by ID from a request parameter
     * @param registry Registry of types eligible for data binding
     * @param enumClass Class of the enum
     */
    private registerEnum(PropertyEditorRegistry registry, Class<? extends Enum<?>> enumClass) {

        boolean hasRequiredMethod = enumClass.metaClass.methods.any {MetaMethod method ->
            method.isStatic() && method.name == REQUIRED_METHOD_NAME && method.parameterTypes.size() == 1
        }

        if (!hasRequiredMethod) {
            throw new MissingMethodException(REQUIRED_METHOD_NAME, enumClass, [String].toArray())
        }
        registry.registerCustomEditor(enumClass, new EnumEditor(enumClass))
    }
}

Step 3: Make Spring aware of the registry above by defining the following Spring bean in grails-app/conf/spring/resources.grooovy

customPropertyEditorRegistrar(CustomPropertyEditorRegistrar)

Answer 2

So the default Databinding binds on the Enum name and not a separately defined property of the Enum. You can either create your own PropertyEditor as you have mentioned or do a work-around similar to this:

class MyCommand {
 String ratingId
    Rating getRating() {
     return Rating.getEnumFromId(this.ratingId)
    }
    static constraints = {
     ratingId(validator:{val, obj -> Rating.getEnumFromId(val) != null })
    }
}