Associaction.Find when associations is NULL

[bkmeneguello]

Discussed in #5649

^{Originally posted by othane August 25, 2022}
Hi, I have an account table, it has an optional 1 to 1 mapping to a subscription table via fk_account_id on the subscription. Something like this:

type Account struct {
	ID            uint   `json:"-" gorm:"primarykey"`
	Subscription    *Subscription `json:",omitempty"`
}

type Subscription struct {
	Status         string
	AccountID int      `json:"-"`
	Account   *Account `json:",omitempty"`
}

If I try to load these nested relationship I can do it either using the Preload/Join initially or lazy load them later if they are missing. Something like this:

        var account Account
	err = a.GormDB.
		Joins("Subscription", a.GormDB.Where(models.Subscription{Status: "active"})). // Preload works here too
		Where(models.Account{ID: id}).
		First(&account).
		Error
	if err != nil {
		log.Println(err)
		return
	}

later in the code if I need the subscription I test if it was loaded and if not I lazy load it

		// Subscription probably not preloaded, lazy loading them now
		err = a.GormDB.Model(&acc).
			Where("status in ?", []string{"active"}).
			Association("Subscription").
			Find(&acc.Subscription)
		if err != nil {
			log.Printf("Error loading subscription")
			return err
		}

Assuming there is a subscription associated with the account this all works great. But if there is no subscription related to the account rather than the account.Subscription == nil being true it is a struct with an empty value which can be hard to tell a part form an empty row in the table.

Worse still when I call the Association("Subscription").Find(&acc.Subscription) above to lazy load it (this could happen because I did not join it originally, or if the Join would set the Subscription to nil like I was expecting) ... But this still just sets the acc.Subscription to be a empty struct, not nil and not returning a gorm.ErrRecordNotFound error.

So my 2 questions are:

1. How to tell if an association is invalid, ie the reverse look up for a subscription above does not exist either when lazy loading or when using the join/preload?
2. How should you detect later if an association is not yet loaded so you can lazy load when your in a another function the requires it?

Thanks for any help with this !!