How can I append to a many-to-many relation in Gorm without upserting the already-existing associated rows?

When using GORM to manage databases, handling many-to-many relationships is a common requirement. Many-to-many relationships are typically implemented through a join table, which usually contains foreign keys referencing both main tables. When adding new associations to an existing many-to-many relationship without affecting or deleting existing associations, we can use specific methods provided by GORM to achieve this.

Here is a specific example. Suppose we have two models, User and Role, which have a many-to-many relationship managed through a join table user_roles:

go
type User struct {
    gorm.Model
    Name  string
    Roles []Role `gorm:"many2many:user_roles;"`
}

type Role struct {
    gorm.Model
    Name string
}

Suppose we want to add a new role to a user without affecting their existing roles; we can use the Append method.

go
func AddRoleToUser(db *gorm.DB, userID uint, roleID uint) error {
    // First, retrieve instances of the user and role
    var user User
    var role Role

    // Check if the user exists
    if err := db.First(&user, userID).Error; err != nil {
        return err
    }

    // Check if the role exists
    if err := db.First(&role, roleID).Error; err != nil {
        return err
    }

    // Use Append to add the role to the user's Roles association
    if err := db.Model(&user).Association("Roles").Append(&role); err != nil {
        return err
    }

    return nil
}

In this example, we first load instances of the user and role. Then, we use db.Model(&user).Association("Roles").Append(&role) to append a new role to the user's roles list. The Append method ensures that existing association data is not deleted or modified; it simply adds new rows to the join table user_roles. This approach is suitable for scenarios where existing data must remain unchanged, and only new elements are added to the relationship. Using the Append method ensures the integrity and accuracy of the database.