assets/js/vendor/backbone-relational.js
function( relation ) {
			var exists = _.any( this._reverseRelations, function( rel ) {
				return _.all( relation || [], function( val, key ) {
					return val === rel[ key ];
				});
			});

			if ( !exists && relation.model && relation.type ) {
				this._reverseRelations.push( relation );
				this._addRelation( relation.model, relation );
				this.retroFitRelation( relation );
			}
		}
function( relation ) {
			var exists = _.any( this._reverseRelations, function( rel ) {
				return _.all( relation || [], function( val, key ) {
					return val === rel[ key ];
				});
			});

			if ( !exists && relation.model && relation.type ) {
				this._reverseRelations.push( relation );
				this._addRelation( relation.model, relation );
				this.retroFitRelation( relation );
			}
		}
Add a reverse relation. Is added to the 'relations' property on model's prototype, and to
existing instances of 'model' in the store as well.
@param {Object} relation
@param {Backbone.RelationalModel} relation.model
@param {String} relation.type
@param {String} relation.key
@param {String|Object} relation.relatedModel