code: public function storeNewParent() {
      if ( $this->isDirty($this->getParentColumnName()) && ($this->exists || !$this->isRoot()) )
        static::$moveToNewParentId = $this->getParentId();
      else
        static::$moveToNewParentId = FALSE;
    }
docs: Store the parent_id if the attribute is modified so as we are able to move
  the node to this new parent after saving.
  
  @return void