Is there a standard way to mock Django models? - django

Question

I have a model called Pdb:
class Pdb(models.Model):
id = models.TextField(primary_key=True)
title = models.TextField()
It is in a one-to-many relationship with the model Residue:
class Residue(models.Model):
id = models.TextField(primary_key=True)
name = models.TextField()
pdb = models.ForeignKey(Pdb)
Unit tesing Pdb is fine:
def test_can_create_pdb(self):
pdb = Pdb(pk="1XXY", title="The PDB Title")
pdb.save()
self.assertEqual(Pdb.objects.all().count(), 1)
retrieved_pdb = Pdb.objects.first()
self.assertEqual(retrieved_pdb, pdb)
When I unit test Residue I just want to use a mock Pdb object:
def test_can_create_residue(self):
pdb = Mock(Pdb)
residue = Residue(pk="1RRRA1", name="VAL", pdb=mock_pdb)
residue.save()
But this fails because it needs some attribute called _state:
AttributeError: Mock object has no attribute '_state'
So I keep adding mock attributes to make it look like a real model, but eventually I get:
django.db.utils.ConnectionDoesNotExist: The connection db doesn't exist
I don't know how to mock the actual call to the database. Is there a standard way to do this? I really don't want to have to actually create a Pdb record in the test database because then the test won't be isolated.
Is there an established best practices way to do this?
Most of the SF and google results I get for this relate to mocking particular methods of a model. Any help would be appreciated.

Answer 1

You are not strictly unit testing here as you are involving the database, I would call that integration testing, but that is another very heated debate!
My suggestion would be to have your wrapping test class inherit from django.test.TestCase. If you are that concerned about each individual test case being completely isolated then you can just create multiple classes with a test method per class.
It might also be worth reconsidering if these tests need writing at all, as they appear to just be validating that the framework is working.

Answer 2

Oh, I managed to solve this with a library called 'mixer'...
from mixer.backend.django import mixer
def test_can_create_residue(self):
mock_pdb = mixer.blend(Pdb)
residue = Residue(pk="1RRRA1", name="VAL", pdb=mock_pdb)
residue.save()
Still think django should provide a native way to do this though. It already provides a lot of testing tools - this feels like a major part of proper unit testing.

Answer 3

I'm not sure exactly what you mean by mocking Django models. The simplest option for writing a test that requires some model objects is to use a test fixture. It's basically a YAML file that gets loaded into a database table before your test runs.
In your answer you mentioned mixer, which looks like a library for randomly generating those test fixtures.
Those are fine tools, but they still require database access, and they're a lot slower than pure unit tests. If you want to completely mock out the database access, try Django mock queries. It completely mocks out the database access layer, so it's very fast, and you don't have to worry about foreign keys. I use it when I want to test some complex code that has simple database access. If the database access has some complicated query conditions, then I stick with the real database.
Full disclosure: I'm a minor contributor to the Django mock queries project.