How can one assert in Django that a model field has already been populated from the DB?

Question

In Django, is there an easy way to test that a model field on an object has already been queried from the database (e.g. an object coming from a foreign-key relationship)?

I would like to make an assertion like this in one of my tests to ensure that accessing a particular attribute on one of my objects won't trigger an additional database query.

Answer 1

In the particular case of a ForeignKey, you can check the existence of the _FOO_cache attribute. For instance, if your Employee object has a ForeignKey to Company, then if my_employee.company is populated then my_employee._company_cache will exist, so you can do hasattr(my_employee, '_company_cache').

Answer 2

You can use TestCase.assertNumQueries.

To check that obj.accessor doesn't hit the database:

# context manager version
with self.assertNumQueries(0):
  obj.accessor

# function version
self.assertNumQueries(0, lambda: obj.accessor)