Testing Django model fields with 'auto_now'

auto_now_test_ex.md

class MyModel(models.Model):
  time_field = models.DateTimeField(auto_now=True)

This makes testing difficult, because the created field will always be the current time. If you try changeing it with

myobject.time_field = _sometime
myobject.save()

it will get reset to the current time when saving.

Here's how to change the time that it gets set to in a test:

testtime = datetime.strptime('2015-10-31', '%Y-%m-%d')
with mock.patch('django.utils.timezone.now') as mock_now:
  mock_now.return_value = testtime
  MyModel.objects.create()

When the with block is over, further models will be created with current time.