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 );
  				
  				if ( !relation.model.prototype.relations ) {
  					relation.model.prototype.relations = [];
  				}
  				relation.model.prototype.relations.push( 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; required properties:
  - model, type, key and relatedModel