IntegrityError with generic relations, unique_together, and inline admin forms

[rutt]

Using the models and admin below, create a new GenericAttachment object using the inline form on the Parent change page. Then create another one with the same slug. The result is an IntegrityError where "columns content_type_id, object_id, slug are not unique". This works fine for Attachment objects.

Models:

class Parent(models.Model):
    name = models.CharField(max_length=200)
    
class Attachment(models.Model):
    name = models.CharField(max_length=200)
    parent = models.ForeignKey(Parent)
    slug = models.SlugField()
    
    class Meta:
        unique_together = ['parent', 'slug']
    
class GenericAttachment(models.Model):
    name = models.CharField(max_length=200)
    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    parent = generic.GenericForeignKey()
    slug = models.SlugField()
    
    class Meta:
        unique_together = ['content_type', 'object_id', 'slug']

Admin:

class GenericAttachmentInline(generic.GenericTabularInline):
    model = GenericAttachment
    
class ParentAdmin(admin.ModelAdmin):
    inlines = [AttachmentInline, GenericAttachmentInline]

admin.site.register(Attachment, admin.ModelAdmin)
admin.site.register(GenericAttachment, admin.ModelAdmin)
admin.site.register(Parent, ParentAdmin)

[rutt]

See also #10770

[mrts]

Duplicate of #9493 .

[Jacob]

Milestone 1.1 deleted