or even smaller and simple:
```javascript
function simplePolymorphicRelation(attributes, relation) {
     var key = relation.key;
     return Models[attributes[key+ '_type'] || this.get(key+ '_type')];
};

Models.Job = Backbone.AssociatedModel.extend({
  relations: [
    {
      type: Backbone.One,
      key: 'organizable',
      relatedModel: simplePolymorphicRelation
    }
  ]
});
```