IDs of child table is invalid when update with FullSaveAssociations mode in MySQL

[isutare412]

GORM Playground Link

go-gorm/playground#805

Description

Environment

• MySQL

Symptom

// Create app
appInitial := &App{
	Name: "foo",
	Envs: []*Env{
		{Key: "KEY1", Value: "value1"},
	},
}
DB.Create(appInitial)
t.Logf("appInitial: %+v", appInitial)
for i, env := range appInitial.Envs {
	t.Logf("  env[%d]: %+v", i, env)
}

// Update app
appUpdated := &App{
	ID:   appInitial.ID,
	Name: "foo",
	Envs: []*Env{
		{Key: "KEY1", Value: "value1-updated"}, // KEY1 should be updated
		{Key: "KEY2", Value: "value2-added"},  // KEY2 should be added
	},
}
DB.Session(&gorm.Session{FullSaveAssociations: true}).Updates(appUpdated)
t.Logf("appUpdated: %+v", appUpdated)
for i, env := range appUpdated.Envs {
	t.Logf("  env[%d]: %+v", i, env)
}

// Get latest app from database
appFetched := &App{ID: appInitial.ID}
DB.Preload("Envs").First(appFetched)
t.Logf("appFetched: %+v", appFetched)
for i, env := range appFetched.Envs {
	t.Logf("  env[%d]: %+v", i, env)
}

for i := range appFetched.Envs {
	envFetched := appFetched.Envs[i]
	envUpdated := appUpdated.Envs[i]

	if envFetched.Key != envUpdated.Key {
		t.Errorf("Expected env key %s, got %s", envFetched.Key, envUpdated.Key)
	}
	if envFetched.Value != envUpdated.Value {
		t.Errorf("Expected env value %s, got %s", envFetched.Value, envUpdated.Value)
	}
	if envFetched.ID != envUpdated.ID {
		t.Errorf("Expected env ID %d, got %d", envFetched.ID, envUpdated.ID)
	}
}

1. App has many Envs
2. Create an app with envs
3. Update the app with FullSaveAssociations mode.
4. The updated app's envs have different IDs compared to database.

IDs of child table after Update() is not same as those saved in database.
As you see, IDs in database is 1, 2 but IDs of updated results are 2, 3.

appUpdated: &{ID:1 Name:foo Envs:[0x140002a3e90 0x140002a3ec0]}
  env[0]: &{ID:2 AppID:1 Key:KEY1 Value:value1-updated}
  env[1]: &{ID:3 AppID:1 Key:KEY2 Value:value2-added}
appFetched: &{ID:1 Name:foo Envs:[0x140002f28a0 0x140002f2930]}
  env[0]: &{ID:1 AppID:1 Key:KEY1 Value:value1-updated}
  env[1]: &{ID:2 AppID:1 Key:KEY2 Value:value2-added}
Expected env ID 1, got 2
Expected env ID 2, got 3
FAIL