Polymorphic relationship

[rabc]

What is the best way to define a polymorphic relationship? I created the models and they are inheriting from the same base class, but then how do I define a ToOne relationship?

I tried using the base class, like (where Group is my base class):

override open class var fields: [Field] {
        return fieldsFromDictionary([
            "group": ToOneRelationship(Group.self)
            ])
    }

But then the deserializer gets the resourceType from the base class, even if I register only the child class.

[markst]

@rabc should be achievable using branch on this PR: #154