IntegrityError when creating multiple inline objects with ForeignKey and unique_together

[rutt]

Using the models below, enter two Attachment objects with the same slug on the Parent change page, and then save. The result is an IntegrityError where "columns parent_id, slug are not unique". If the two Attachment objects are created and saved individually, the admin shows the expected form error: "Attachment with this Parent and Slug already exists."

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']

Admin:

class ParentAdmin(admin.ModelAdmin):
    inlines = [AttachmentInline]

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

[rutt]

See also #10769

[mrts]

Duplicate of #9493 .

[Jacob]

Milestone 1.1 deleted