Attempt to create object with repeated value on a custom PK raises IntegrityError on wrong field

[Evandro Myller]

I have this model Product, child of an abstract model which defines a created field with auto_now_add set. All the CRUD works just fine so far.

The problem happens when I add a custom primary key to Product: If I try to create a Product object through the admin giving the reference field a repeated value, expecting to see a nice validation error message saying that another object with that reference already exists, I get an IntegrityError stating that the created field cannot be NULL (traceback attached) -- nothing even about the custom primary key.

Notes:

• I noticed the exception is raised from an UPDATE query, which is really odd since I'm posting data from the admin add view.
• Product(reference=x).save() raises the same exception, but Product.objects.create(reference=x raises the expected exception (IntegrityError about the PK's UNIQUE constraint), which proves that it's not a problem on the admin.

[Evandro Myller]

full traceback

[Evandro Myller]

example models