Gorm Preload don't works with spec ID in Model()

type Message struct {
    gorm.Model
    Attachment Attachment
}

type Attachment struct {
    gorm.Model
    MessageID  uint
}

Now, if you try this:

db.Model(&models.Message{}).Where(&models.Message{
    Model: gorm.Model{
        ID: 1,
    },
}).Preload("Attachment")).First(&message)

All works perfectly, and Attachment is preloaded

But if you try this:

db.Model(&models.Message{
        Model: gorm.Model{
        ID: body.MessageID,
    },
}).Preload("Attachment").First(&message)

Message will finded, but attachment will not preload. Is it a bug or feature?

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Comment From: twocs

It's hard to tell because you forgot to add a Playground reproducible, but I think you are pointing out a difference in behaviour between the docs and ID in gorm.Model. The docs say:

When the destination object has a primary value, the primary key will be used to build the condition, for example:

var user = User{ID: 10} 
db.First(&user) 
// SELECT * FROM users WHERE id = 10; 
var result User 
db.Model(User{ID: 10}).First(&result) 
// SELECT * FROM users WHERE id = 10 AND deleted_at IS NULL;

However, when we use gorm.Model, the ID does not get used. Does that mean it's not a "primary value"?

var user = User{Model: gorm.Model{ID: 10}} 
db.First(&user) 
// SELECT * FROM `users` WHERE `users`.`id` = 10 AND `users.``deleted_at` IS NULL ORDER BY `users`.`id` LIMIT 1;
var result User 
db.Model(Model: gorm.Model{ID: 10}).First(&result) 
// SELECT * FROM `users` WHERE `users.``deleted_at` IS NULL ORDER BY `users`.`id` LIMIT 1;

Personally I would discourage modifying the object in the Model func, because it might have unexpected results. For example, if you do an updates but have already preloaded other fields, gorm might like to update your preload tables.

Comment From: github-actions[bot]