db access wrapper for Go project

Question

I'm adding db access to my go program, it looks like this:

package db_access

import (
    "database/sql"
    "fmt"
    "my_database"
)

type TheType struct {
    Name string 
}

func LoadTheTypeFromDb(db *sql.DB, id uint) (*TheType, error) {
    query := fmt.Sprintf(
        "SELECT %s FROM %s WHERE %s = ?",
        my_database.NAME,
        my_database.TABLE,
        my_database.ID,
    )

    var theType TheType

    err := db.QueryRow(query, id).Scan(&theType.Name)

    switch {
    case err == sql.ErrNoRows:
        n := my_database.NewNotFoundError(err)
        return nil, &n
    case err != nil:
        return nil, err
    default:
        return &theType, nil
    }
}

And some part of the my_database package:

package my_database

import (
    "database/sql"
    _ "github.com/go-sql-driver/mysql"
)

func GetDatabaseInstance(connectionString string) (*sql.DB, error) {
    db, err := sql.Open("mysql", connectionString)

    if err != nil {
        c := newConnectionError(err)
        return nil, &c
    }

    err = db.Ping()

    if err != nil {
        c := newConnectionError(err)
        return nil, &c
    }

    return db, nil
}

And the custom errors:

package my_database

import (
    "fmt"
    "reflect"
)

type ConnectionError struct {
    errorMessage string
}

func newConnectionError(err error) ConnectionError {
    return ConnectionError{fmt.Sprintf("Connection error %s %s", reflect.TypeOf(err), err)}
}

func (e *ConnectionError) Error() string {
    return e.errorMessage
}

type QueryError struct {
    errorMessage string
}

func newQueryError(err error) QueryError {
    return QueryError{fmt.Sprintf("Query error %s %s", reflect.TypeOf(err), err)}
}

func (e *QueryError) Error() string {
    return e.errorMessage
}

type NotFoundError struct {
    errorMessage string
}

func NewNotFoundError(err error) NotFoundError {
    return NotFoundError{fmt.Sprintf("Not found - %s", err)}
}

func (e *NotFoundError) Error() string {
    return e.errorMessage
}

• I add a custom NotFound error, because when the row is not found, I don't want to leak sql.ErrNoRows outside, is that ok? I don't like that the constructor for NotFound error is public, unlike all other custom errors, furthermore, I think I would need to add that switch statement to every Load* method … also not so good.
• The custom error code is very repetitive, is there a way to clean that up?
• Any other comments are much appreciated.

Answer 1

Why don't you want to leak sql.ErrNoRows outside? I find it quite explicit.

It seems that you are wrapping your errors with some context: there is a very nice package for this: https://godoc.org/github.com/pkg/errors

Using it, your code would look like this:

package db_access

import (
    "database/sql"
    "fmt"
    "my_database"
)

type TheType struct {
    Name string 
}

// uses named returns
func LoadTheTypeFromDb(db *sql.DB, id uint) (row TheType, err error) {
    query := fmt.Sprintf(
        "SELECT %s FROM %s WHERE %s = ?",
        my_database.NAME,
        my_database.TABLE,
        my_database.ID,
    )

    err = db.QueryRow(query, id).Scan(&row.Name)
    return
}

And your "my_database"

package my_database

import (
    "database/sql"
    _ "github.com/go-sql-driver/mysql"
    "github.com/pkg/errors"
)

func GetDatabaseInstance(connectionString string) (db *sql.DB, err error) {
    db, err = sql.Open("mysql", connectionString)

    if err != nil {
        err = errors.Wrap("open failed", err)
        return
    }

    err = db.Ping()
    // if err is nil, errors.Wrap returns nil
    err = errors.Wrap("ping failed", err)

    return
}

Your custom error file should not be needed anymore!