Django Model Constraint Condition Using Field From Inherited Class - Is It Possible?

Question

I'd like to use a field from a parent class as a constraint condition in a child class.

models.py

class ParentClass(object):
    ...
    is_public = models.BooleanField(default=False)


class ChildClass(ParentClass):
    ...
    price = models.DecimalField(max_digits=6, decimal_places=2, null=True)
    class Meta:
        constraints = [
            models.CheckConstraint(
                check=Q(price__isnull=True) & Q(is_public=True), # <- here
                name='price_exists_check',
            )
        ]

When I attempt to migrate, I see this error in my terminal:

myapp.ChildClass: (models.E016) 'constraints' refers to field 'is_public'
  which is not local to model 'ChildClass'.
  HINT: This issue may be caused by multi-table inheritance.

It's obvious why I am seeing this error (is_public field lives in ParentClass). My question is, is it simply not possible then, or can I refactor something?

What is my end-goal?

To not let an instance of ChildClass is_pulic change to True if the price is null. I'd like to enforce this at the database level.

Is there a way and if so, what needs to change?

Does marking the parent as an abstract class help? See https://stackoverflow.com/a/36824386/530160 Or do you need the parent class to be a concrete class? — Nick ODell, Oct 20 '21 at 20:10

score 2 · Accepted Answer · answered Oct 20 '21 at 20:55

My question is, is it simply not possible then, or can I refactor something?

I tracked down the original commit which introduced the error message you're seeing, and the bug it was fixing. The documentation on model inheritance is also helpful for understanding this.

Here's how I understand the issue:

If you create a Django model which inherits from another model, and that model is not abstract, then Django creates a foreign key to the parent table, rather than repeat all of the fields from the parent model in the child table. So when you create a ChildClass object, that creates both a row in the ChildClass table and the ParentClass table.

You can't create a CHECK constraint which references multiple tables. (As far as I know.) Therefore, Django forbids you from creating this constraint.

Therefore, you have the following options:

Enforce it at the ORM layer. In the clean() method, check if your constraint is satisfied. (Documentation.) This won't prevent violation of the constraint if a non-Django program modifies the database.
Make the parent class abstract. You've already said this is not workable.

Make a third class, which both inherit from. Create a base class like this:

            -------------
            | BaseClass |
            -------------
            |           |
            V           V
---------------       --------------
| ParentClass |       | ChildClass |
---------------       --------------

Make the BaseClass abstract, and the ParentClass and ChildClass concrete. This allows you to use a constraint, because the ChildClass data is inside just one table now.

Django Model Constraint Condition Using Field From Inherited Class - Is It Possible?

1 Answers1