Support callables as default values in django model fields

[pakal]

When using django-model-utils TimeStampedModel subclass, which uses default=now callables in "created" and "modified" fields, generation of model instances (with _fill_optional=True) via baker fails:

  File "...site-packages\django\utils\dateparse.py", line 107, in parse_datetime
    match = datetime_re.match(value)   # here value is function "now"
TypeError: expected string or bytes-like object

This is probably because bakery uses the default value as is, instead of calling it (when relevant) to get the actual default value.

Versions

Python: 3.7.9
Django: 3.1.2
Model Bakery: 1.2.0

[timjklein36]

I was able to reproduce this issue with the following setup:

Versions

Python: 3.7.5

Django==3.1.2
django-model-utils==4.0.0
model-bakery==1.2.0

models.py

from django.db import models
from django.utils.timezone import now
from model_utils.models import TimeStampedModel


class Dummy(TimeStampedModel):
    pass


class SimpleDummy(models.Model):
    dt = models.DateTimeField(default=now)

tests.py

from datetime import datetime

from django.test import TestCase
from model_bakery import baker


class DummyTest(TestCase):

    def test_bakery_default_timestamped(self):
        dummy = baker.make('my_app.Dummy', _fill_optional=True)
        self.assertTrue(isinstance(dummy.created, datetime))

    def test_bakery_default_simple(self):
        dummy = baker.make('my_app.SimpleDummy', _fill_optional=True)
        self.assertTrue(isinstance(dummy.dt, datetime))

Here, baker.make works for SimpleDummy but fails for Dummy (the model extending TimeStampedModel).

The issue here is that model_bakery.generators has no default generator for the class of the field created on TimeStampedModel because it extends models.DateTimeField:

model_utils.fields.py

class AutoCreatedField(models.DateTimeField):
    ...

This is definitely a bug one way or another, but there are multiple things going on here.

The model_bakery code falls through the check in this block when generating a value for the field:

model_bakery.baker.py

if field.name in self.attr_mapping:
    generator = self.attr_mapping[field.name]
elif getattr(field, "choices"):
    generator = random_gen.gen_from_choices(field.choices)
elif is_content_type_fk:
    generator = self.type_mapping[contenttypes.models.ContentType]
elif generators.get(field.__class__):
    generator = generators.get(field.__class__)
elif field.__class__ in self.type_mapping:
    generator = self.type_mapping[field.__class__]
elif field.has_default():
    return field.default
else:
    raise TypeError("%s is not supported by baker." % field.__class__)

...which means that field.has_default() evaluates to True and the default value is returned, in this case django.utils.timezone.now which is a function. It is definitely evident that this should take into account if the default value is a callable and, if so, call it.

@berinhard
Should the conditional block be reorderd (or broken up) so that if there is a default it is always used?

The reason why SimpleDummy.dt, from above, works is because it never gets to the field.default check (because the elif field.__class__ in self.type_mapping check passes when it retrieves models.DateTimeField from the default type mapping). This means that it does not use the default value in the case where the field class has a generator available. Should this be true?

[berinhard]

Fixed by #117

[pakal]

Awesome, thanks !