Polymorphic associations with Backbone Associations v0.5.1+

poly_associations.js

var Models = {};
var findPolyMorphicType = Models.findPolyMorphicType = function (relation, attributes) {
    var key = relation.key + '_type';
    return Models[attributes[key] || this.get(key)];
};

Models.Job = Backbone.AssociatedModel.extend({
    relations:[
        {
            type:Backbone.One,
            key:'organizable',
            relatedModel:findPolyMorphicType
        }
    ]

});

Models.Comment = Backbone.AssociatedModel.extend({
    relations:[
        {
            type:Backbone.One,
            key:'commentable',
            relatedModel:findPolyMorphicType
        }
    ]
});

Models.Company = Backbone.AssociatedModel.extend({});
Models.Department = Backbone.AssociatedModel.extend({});


Models.Article = Backbone.AssociatedModel.extend({});
Models.StatusUpdate = Backbone.AssociatedModel.extend({});

var cjob = new Models.Job({organizable_type:'Company', name:"J1", organizable:{name:"Google"}});
var djob = new Models.Job({organizable_type:'Department', name:"J2", organizable:{name:"Google Reader"}});

var articleComment = new Models.Comment({
    commentable_type:'Article',
    name:"c1",
    commentable:{body:"Wonderful post!"}
});
var statusComment = new Models.Comment({
    commentable_type:'StatusUpdate',
    name:"c2",
    commentable:{body:"Why are you updating your status with pointless crap?"}
});

cjob.get('organizable') instanceof Models.Company === true;
djob.get('organizable') instanceof Models.Department === true;

articleComment.get('commentable') instanceof Models.Article === true;
statusComment.get('commentable') instanceof Models.StatusUpdate === true;

Thanks to @sgrif for API idea, definition and example. @monokrome for API discussions and suggestions

Available on edge version currently. Candidate for v0.5.1 release