Using transaction and simple DB connection in the same struct because of this pattern what can I do?

Question

I found a great example of transactions between Repositories using a Clean Architecture approach.

This guy is using Gorm .

Gorm has the same type for a DB connection and a transaction, example:

var db *gorm.DB
var tx *gorm.DB

I'm a fan of go-pg . But here the types are different (maybe it's even better as is), example:

var db *pg.DB
var tx *pg.Tx

And of course the error is: Cannot use 'tx' (type *Tx) as type *pg.DB

A small reproduction:

package main

import (
    "github.com/go-pg/pg/v10"
)

type Player struct {
    ID   int
    Name string
}

type PlayerRepo struct {
    db       *pg.DB
    teamRepo *TeamRepo
}

type TeamRepo struct {
    db *pg.DB
}

func NewPlayerRepo(db *pg.DB) *PlayerRepo {
    return &PlayerRepo{
        db:       db,
        teamRepo: NewTeamRepo(db),
    }
}

func NewTeamRepo(db *pg.DB) *TeamRepo {
    return &TeamRepo{db: db}
}

func (r *PlayerRepo) Find(id int) (*Player, error) {
    var player Player
    err := r.db.Model(&player).Where("id = ?", id).Select()
    if err != nil {
        return nil, err
    }
    return &player, nil
}

func (r *PlayerRepo) All() ([]*Player, error) {
    // Long code
    return nil, nil
}

func (r *PlayerRepo) Insert() (*Player, error) {
    // Long code
    return nil, nil
}

func (r *PlayerRepo) Update() (*Player, error) {
    // Long code
    return nil, nil
}

func (r *PlayerRepo) Delete() (*Player, error) {
    // Long code
    return nil, nil
}

func (r *PlayerRepo) WithTransaction(txFunc func(*PlayerRepo) error) (err error) {
    tx, _ := r.db.Begin()
    manager := NewPlayerRepo(tx) // <<<--- here the problem! tx is not good here, it's `pg.Tx` not `pg.DB`
    err = txFunc(manager)
    return
}

What can I do to fix this?

Thanks in advance. ❤️

Answer 1

You can define an interface that is already, implicitly implemented by both:

type DB interface {
    Begin() (*Tx, error)
    Close() error
    Context() context.Context
    CopyFrom(r io.Reader, query interface{}, params ...interface{}) (res Result, err error)
    CopyTo(w io.Writer, query interface{}, params ...interface{}) (res Result, err error)
    Exec(query interface{}, params ...interface{}) (Result, error)
    ExecContext(c context.Context, query interface{}, params ...interface{}) (Result, error)
    ExecOne(query interface{}, params ...interface{}) (Result, error)
    ExecOneContext(c context.Context, query interface{}, params ...interface{}) (Result, error)
    Formatter() orm.QueryFormatter
    Model(model ...interface{}) *orm.Query
    ModelContext(c context.Context, model ...interface{}) *orm.Query
    Prepare(q string) (*Stmt, error)
    Query(model interface{}, query interface{}, params ...interface{}) (Result, error)
    QueryContext(c context.Context, model interface{}, query interface{}, params ...interface{}) (Result, error)
    QueryOne(model interface{}, query interface{}, params ...interface{}) (Result, error)
    QueryOneContext(c context.Context, model interface{}, query interface{}, params ...interface{}) (Result, error)
    RunInTransaction(ctx context.Context, fn func(*Tx) error) error
}

NOTE: I only know that the method names match, I didn't bother checking if the signatures also do, if they don't, you'll need to edit the interface accordingly.

You can add a simple "compiler check":

var _ DB = (*pg.DB)(nil)
var _ DB = (*pg.Tx)(nil)

And then you can change the type of the PlayerRepo.db field from *pg.DB to your new DB interface.

type PlayerRepo struct {
    db       DB
    teamRepo *TeamRepo
}

type TeamRepo struct {
    db DB
}

func NewPlayerRepo(db DB) *PlayerRepo {
    return &PlayerRepo{
        db:       db,
        teamRepo: NewTeamRepo(db),
    }
}

func NewTeamRepo(db DB) *TeamRepo {
    return &TeamRepo{db: db}
}


func (r *PlayerRepo) WithTransaction(txFunc func(*PlayerRepo) error) (err error) {
    tx, err := r.db.Begin()
    if err != nil {
        return err
    }
    defer func() {
        // rollback if err; commit if no err
    }()
    manager := NewPlayerRepo(tx)
    err = txFunc(manager)
    return
}

---

If your repo types need to be able to invoke some of the methods that are not common to both pg.DB and pg.Tx , and therefore not defined by the new DB interface, then, one approach would be to retain the original types for such use, for example:

type PlayerRepo struct {
    db       DB
    pg       *pg.DB
    teamRepo *TeamRepo
}

type TeamRepo struct {
    db DB
    pg *pg.DB
}

func NewPlayerRepo(db DB, pg *pg.DB) *PlayerRepo {
    return &PlayerRepo{
        db:       db,
        pg:       pg,
        teamRepo: NewTeamRepo(db, pg),
    }
}

func NewTeamRepo(db DB, pg *pg.DB) *TeamRepo {
    return &TeamRepo{db: db, pg: pg}
}

func (r *PlayerRepo) WithTransaction(txFunc func(*PlayerRepo) error) (err error) {
    tx, err := r.db.Begin()
    if err != nil {
        return err
    }
    defer func() {
        // rollback if err; commit if no err
    }()
    manager := NewPlayerRepo(tx, r.pg)
    err = txFunc(manager)
    return
}

---

Note that if you decide to use orm.DB , which is reasonable, but it is missing some of the methods that you need and that are already implemented by both pg.DB and pg.Tx , then you could embed orm.DB into your custom interface and add only those methods that are missing.

type DB interface {
    Begin() (*Tx, error)
    orm.DB
}