There's a proposal to remove this behavior:
>>> obj.fk = None
ValueError('Cannot assign None: "Obj.fk" does not allow null values.)
(i.e. no more exception if you assign None to a non-nullable foreign key)
Rationale presented on the ticket [1]:
- Conceptually it is a programmer's responsibility to validate the data
assigned to foreign key in a right moment - not necessarily during
assignment to the model field.
- This behavior is not symmetrical to other database-related constraints
as for example unique=True which is deferred to save() method (raising
IntegrityError <https://code.djangoproject.com/wiki/IntegrityError>) or
full_clean() method (raising ValidationError).
- In #13776 <https://code.djangoproject.com/ticket/13776> the decision
was made that a None value for foreign key field with null=False and
corresponding form field with required=False should be valid. It means
that after model instantiation by such a form, the value of a given field
is empty or just unset (not defined). Whatever the state is, it doesn't
matter. It shows only that currently discussed check does *not* prevent
in 100% against having foreign key field with null=False set to None. It
undermines the legitimacy of presence for the discussed check.
Any concerns with this?
Its removal helps fix an edge case that regressed in Django 1.8 where if
you have a model field with blank=False but required=False on the form
field, an empty form value will be silently ignored and won't overwrite a
model instance's value. I think it's a sufficient edge case, however, that
it's not worth backporting this change (if accepted).
[1] https://code.djangoproject.com/ticket/25349
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