docs: function( model ) {
  			var coll = this.getCollection( model );
  
  			if ( coll ) {
  				if ( coll.get( model ) ) {
  					throw new Error( "Cannot instantiate more than one Backbone.RelationalModel with the same id per type!" );
  				}
  
  				var modelColl = model.collection;
  				coll.add( model );
  				model.bind( 'destroy', this.unregister, this );
  				model.collection = modelColl;
  			}
  		}