I’ve got an IntegrityError when I remove the user from django-admin. I uses custom User model, but I didn’t change that much rather than AbrstractUser from django.contrib.auth.models. The error was like this:

IntegrityError: (1048, "Column 'user_id' cannot be null")

I couldn’t find the case at all. I asussemed that the error occurs from MySQL, I checked all table schema one by one and finally found the reason.

One of the model includes User ForeignKey with on_delete=models.SET_DEFAULTS which is fine because when remove the user, it will set the default value, None in this case. This model is the record for the signing policy so it also includes Policy ForeignKey. Then these fields sets as unique_together that the database create a unique key based on these two fields. When removed the user, the field followed on_delete action and tried to remove user_id, then the update failed because of the unique_together.

I should be more careful when I use unique_together, Django is too easy to create this kind of relationship on the code, especially. With great power comes great responsibility!

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