code: function(snap/*, prevChild*/) {
            var rec = this.$getRecord(snap.key);
            if( angular.isObject(rec) ) {
              rec.$priority = snap.getPriority();
              return true;
            }
            return false;
          }
docs: Called whenever an item changes order (moves) on the server.
  This method should set $priority to the updated value and return true if
  the record should actually be moved. It should not actually apply the move
  operation.
  
  If this method returns false, then the record will not be moved in the array
  and no $watch listeners will be notified. (When true, $$process is invoked
  which invokes $$notify)
  
  @param {object} snap a Firebase snapshot
  @param {string} prevChild
  @protected