public function equal($item)
    {
        if ($item instanceof static) {
            $v = $this->tableName == $item->tableName;
            return
                $this->tableName == $item->tableName &&
                $this->parentAlias == $item->parentAlias &&
                get_class($this->joinField) === get_class($item->joinField);
        }

        return false;
    }