code: function( model, options ) {
  			if ( this.related ) {
  				return;
  			}
  			options = this.sanitizeOptions( options );
  			
  			var item = this.keyContents;
  			if ( item || item === 0 ) { // since 0 can be a valid `id` as well
  				var id = Backbone.Relational.store.resolveIdForItem( this.relatedModel, item );
  				if ( !_.isNull( id ) && model.id === id ) {
  					this.addRelated( model, options );
  				}
  			}
  		}
docs: If a new 'this.relatedModel' appears in the 'store', try to match it to the last set 'keyContents'