I'm using Django 1.8.4 in my dev machine using Sqlite and I have these models:
class ModelA(Model):
field_a = CharField(verbose_name='a', max_length=20)
field_b = CharField(verbose_name='b', max_length=20)
class Meta:
unique_together = ('field_a', 'field_b',)
class ModelB(Model):
field_c = CharField(verbose_name='c', max_length=20)
field_d = ForeignKey(ModelA, verbose_name='d', null=True, blank=True)
class Meta:
unique_together = ('field_c', 'field_d',)
I've run proper migration and registered them in the Django Admin. So, using the Admin I've done this tests:
- I'm able to create ModelA records and Django prohibits me from creating duplicate records - as expected!
- I'm not able to create identical ModelB records when field_b is not empty
- But, I'm able to create identical ModelB records, when using field_d as empty
My question is: How do I apply unique_together for nullable ForeignKey?
The most recent answer I found for this problem has 5 year... I do think Django have evolved and the issue may not be the same.
UPDATE: previous version of my answer was functional but had bad design, this one takes in account some of the comments and other answers.
In SQL NULL does not equal NULL. This means if you have two objects where
field_d == None and field_c == "somestring"
they are not equal, so you can create both.You can override
Model.clean
to add your check:If used outside of forms you have to call
full_clean
orvalidate_unique
.Take care to handle the race condition though.
@ivan, I don't think that there's a simple way for django to manage this situation. You need to think of all creation and update operations that don't always come from a form. Also, you should think of race conditions...
And because you don't force this logic on DB level, it's possible that there actually will be doubled records and you should check it while querying results.
And about your solution, it can be good for form, but I don't expect that save method can raise ValidationError.
If it's possible then it's better to delegate this logic to DB. In this particular case, you can use two partial indexes. There's a similar question on StackOverflow - Create unique constraint with null columns
So you can create Django migration, that adds two partial indexes to your DB
Example: