assets/js/vendor/backbone-relational.js
function( model ) {
			var coll = this.getCollection( model );

			if ( coll ) {
				var modelColl = model.collection;
				coll.add( model );
				this.listenTo( model, 'destroy', this.unregister, this );
				this.listenTo( model, 'relational:unregister', this.unregister, this );
				model.collection = modelColl;
			}
		}
function( model ) {
			var coll = this.getCollection( model );

			if ( coll ) {
				var modelColl = model.collection;
				coll.add( model );
				this.listenTo( model, 'destroy', this.unregister, this );
				this.listenTo( model, 'relational:unregister', this.unregister, this );
				model.collection = modelColl;
			}
		}
Add a 'model' to its appropriate collection. Retain the original contents of 'model.collection'.
@param {Backbone.RelationalModel} model