Django: constraint with nullable multiple field conditions - sql

Question

I have a model in Django/PostgreSQL project and want to add following constraints to three nullable fields: either all fields are NULL, either all of them aren't.
Here's code (simplified):
class SomeModel(models.Model):
...
a = models.IntegerField(nullable=True)
b = models.DateTimeField(nullable=True)
c = models.DateTimeField(nullable=True)
...
class Meta:
constraints = [
models.CheckConstraint(
check=(
(Q(a__isnull=True) & Q(b__isnull=True) & Q(c__isnull=True)) |
(Q(a__isnull=False) & Q(b__isnull=False) & Q(c__isnull=False))
)
)
]
If I understand correctly, I've just described two possible states of those three fields. First is "all three are NULL", second is "none of them are NULL".
But what I got, actually, is "none of them can be NULL". Django admin panel insists on filling all of the fields, they all are mandatory. How can I fix this behaviour? Thanks!

Answer 1

This is not due to the constraints, you should specify blank=True [Django-doc] when you want to allow to leave form fields blank, and thus default to None/NULL:
class SomeModel(models.Model):
# …
a = models.IntegerField(null=True, blank=True)
b = models.DateTimeField(null=True, blank=True)
c = models.DateTimeField(null=True, blank=True)
class Meta:
constraints = [
models.CheckConstraint(
check=Q(a=None, b=None, c=None) |
Q(a__isnull=False, b__isnull=False, c__isnull=False),
name='all_or_none_null'
)
]