判断表是否join

            $joins       = $query->getQuery()->joins;
            $isJoined = false;
            if ($joins) {
                foreach ($joins as $join) {
                    if ($join->table == 'x') {
                        $isCarJoined = true;
                        break;
                    }
                }
            }
            if (!$isJoined) {
                $query->leftJoin('x', 'x.id', '=', 'y.x_id');
            }