code: 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 );
  				
  				var addRelation = function( model, relation ) {
  					if ( !model.prototype.relations ) {
  						model.prototype.relations = [];
  					}
  					model.prototype.relations.push( relation );
  					
  					_.each( model._subModels, function( subModel ) {
  							addRelation( subModel, relation );
  						}, this );
  				};
  				
  				addRelation( relation.model, relation );
  				
  				this.retroFitRelation( relation );
  			}
  		}
docs: 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