How to compare fields within a Constraint

Using Django Using the ORM
1

I am migrating some custom validation code into declarative Constraints.

As a minimal example:

class MyModel(Model):
    a = IntegerField()
    b = IntegerField()

I’d like to create a CheckConstraint that enforces that a > b.

The examples in the docs show only basic comparisons to static values, I was wondering if comparing between columns was also possible?

The possible Field lookups I’ve been reading about as options to a Q object:

2

It appears that F expressions may be the tool to use. If so, it might be helpful to add at least one small example to the Constraint docs for this.

3

It looks like the solution is:

    class Meta:
        constraints = [
            CheckConstraint(
                name="%(app_label)s_%(class)s_a_less_than_b",
                check=Q(a__lt=F("b")),
                violation_error_message="A must be less than B",
            ),
        ]

I’d still make the suggestion that a small example using F be added to the constraints docs.

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