protected function _tablePrimaryKey()
    {
        $alias = $this->_table->alias();
        $pk = $this->_table->primaryKey();

        if (!is_array($pk)) {
            $pk = [$pk];
        }

        $pk = array_map(function ($key) use ($alias) {
            return "{$alias}.{$key}";
        }, $pk);

        return $pk;
    }