Polymorphic belongsTo in many to many mapping in grails? - mysql

Question

So i know this is possible using a superclass, however, this is very limiting in flexibility. So my question is then, can i use an interface? Something ala.
interface Taggable {
/*Adds tag(s) and returns a list of currently set tags*/
List<String> addTags(String ... tag)
/*Removes tag(s) and returns a list of currently set tags*/
List<String> removeTags(String ... tag)
}
class User implements Taggable {
String username
static hasMany = [tags:Tag]
}
class Tag {
String name
static hasMany = [references:Taggable]
static belongsTo = Taggable
static constraints = {
name(nullable: false, blank: false, unique: true)
}
}
Im interested in a reference back to the object who has the following tag. This object however can't extend a concrete class. Thats why im wondering if this can be done with an interface instead.
So, can it be done?

Answer 1

Hibernate can map an interface - see example. I doubt if Grails supports this in by-convention mapping - but you can try using the mapping annotations from example above, or XML config.
edit: answering a comment question:
On a database level, you have to have a Taggable table for Tag.References to reference with a foreign key.
Discriminator will NOT defeat polymorphism, if it's added automatically - for instance, in table-per-hierarchy mapping, Hibernate/Gorm adds a class field in order to find out a concrete class when reading object from db.
If you map your Taggables to two tables - Taggable part to Taggable and everything else to specific table, referenced 1:1 - all the discriminator work should be done for you by Hibernate.
BTW class field is pretty long - it contains fully qualified class name.
edit 2:
Either way, it's getting pretty complex, and I'd personally go with the approach I suggested in another question:
dynamically query all the classes with Taggable interface for hasMany=[tags:Tag] property;
or, less preferable - to have a hand-crafted child table and a discriminator.