Hi folks,
We've had check constraints for a while now and they're awesome.
I'd like to propose an alternative way to declare check constraints: at the
field level. This sounds like it's duplicating the feature but there are
some advantages that make this distinct from declaring at the model-level
from the meta:
- Colocality: the check rules are close to the field they're concerned
with
- Reusability: Allows for bundling with custom field types
- Good for checks concerned only with the field its declared on, for
multiple fields recommended to continue to use Meta. (Kind of analogous to
unique=True vs UniqueConstraint)
For example:
class Product(Model):
price = DecimalField(..., check=Q(price__gte=0))
...
other fields
...
class Meta:
constraints = [
... declare constraints here that are concerned with multiple
fields...
]
For more complex fields you can then bundle the check for reusability:
class PriceField(DecimalField):
def contribute_to_class(self, ...):
super().contribute_to_class(...)
self.check = Q(**{f'{self.name}__gte': 0})
Some other points:
- To be consistent with model-level check constraints they'd also need
to participate in validate_constraints().
- (Small)PositiveIntegerField already has its own implementation of a
check constraint, enforcing values >- 0 via the private db_check() method.
I think this is an example of how bundling checks can be useful.
- I won't go into implementation alternatives but making use of this
existing db_check() method is one possibility. How participation in
validation would work would still need to be decided upon.
- See this Stupid Django Trick
<https://github.com/shangxiao/stupid-django-tricks/tree/master/column_check_constraints>
for some experimentation with this idea.
Cheers,
David
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