Introduction to Object-relational mapping¶

In go-pg Object-relational mapping support consists of 2 related but separate things:

1. Mapping data between Go structs and SQL tables. See Models.

2. Support for common table relations:

   • Has one.
   • Belongs to.
   • Has many.
   • Many to many.

The second point requires more explanation, because understanding how relations work is necessary to write complex queries using relations that are not supported out-of-the-box.

In the following example we have a User model that has one Profile model:

type Profile struct {
    ID   int
    Lang string
}

// User has one Profile.
type User struct {
    ID        int
    Name      string
    ProfileID int
    Profile   *Profile `pg:"-"`
}

You can select a user with his profile without defining any relations. go-pg is smart enough to map the SQL column profile__id to the Go field User.Profile.ID.

db.Model(&user).
    ColumnExpr("user.*").
    ColumnExpr("profile.id AS profile__id").
    ColumnExpr("profile.lang AS profile__lang").
    Join("LEFT JOIN profiles AS profile ON profile.user_id = user.id").
    Where("user.id = ?", 123).
    Select()

But you can do better by defining a relation:

type User struct {
    ID        int
    Name      string
    ProfileID int
    Profile   *Profile `pg:"rel:has-one"` // note the has-one relation
}

Then the same query can be written as:

db.Model(&user).Relation("Profile").Where("user.id = ?", 123).Select()