fix: many2many association with duplicate belongs to elem

[bsmith-auth0]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

Extends #5473 to many2many models with belongs to associations.

User Case Description

Using the following models that have many2many and belongs to associations:

type Application struct {
	ID        string `gorm:"primarykey"`
	Name      string
	Resources []Resource 
}

type Resource struct {
	ID       string `gorm:"primarykey"`
	Name     string
	OwnerId  string
	Owner    Owner 
}

type Owner struct {
	ID   string `gorm:"primarykey"`
	Name string
}

In cases where two resources share the same Owner, attempting to update an Application using FullSaveAssociations: true will result in an error. GORM tries to insert the same Owner multiple times in a single INSERT command:

INSERT INTO "owner" ("id","name") VALUES ('1','owner-1'),('1','owner-1') ON CONFLICT ("id") DO UPDATE SET "name"="excluded"."name"
ERROR: ON CONFLICT DO UPDATE command cannot affect row a second time (SQLSTATE 21000)

After the fix, GORM will identify and insert only the unique owner(s):

INSERT INTO "owner" ("id","name") VALUES ('1','owner-1') ON CONFLICT ("id") DO UPDATE SET "name"="excluded"."name"