Setting Manager for Django models dynamically

Question

I'm trying to set my custom manager dynamically for given models.

Here is a sample of my code

class CustomManager(models.Manager):
    def active(self):
        return self.filter(name__startswith='c')


def set_custom_manager(model_class, manager=CustomManager(), manager_name='objects'):
    setattr(model_class, manager_name, manager)
    return model_class


class Facility(BaseModel):
    name = models.CharField(_("Name"), max_length=255)
    icon = models.ImageField(upload_to='location/facilities/')
    order = models.PositiveIntegerField(_("Order"), default=0)

    class Meta:
        verbose_name = _("Facility")
        verbose_name_plural = _("Facilities")
        ordering = ['order', 'id']

    def __str__(self):
        return self.name

set_custom_manager(Facility, CustomManager())

If I check manager class name of Facility instance, it's returning true

>>> Facility.objects.__class__
<class 'apps.chargers.models.CustomManager'>

But if I try to use default methods of models.Manager, it's returning error

Facility.objects.all()
Traceback (most recent call last):
  File "/opt/homebrew/Cellar/[email protected]/3.10.14/Frameworks/Python.framework/Versions/3.10/lib/python3.10/code.py", line 90, in runcode
    exec(code, self.locals)
  File "<console>", line 1, in <module>
  File "/Users/njr/PycharmProjects/iwatt-backend/venv/lib/python3.10/site-packages/django/db/models/query.py", line 376, in __repr__
    data = list(self[: REPR_OUTPUT_SIZE + 1])
  File "/Users/njr/PycharmProjects/iwatt-backend/venv/lib/python3.10/site-packages/django/db/models/query.py", line 400, in __iter__
    self._fetch_all()
  File "/Users/njr/PycharmProjects/iwatt-backend/venv/lib/python3.10/site-packages/django/db/models/query.py", line 1928, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "/Users/njr/PycharmProjects/iwatt-backend/venv/lib/python3.10/site-packages/django/db/models/query.py", line 99, in __iter__
    model_cls = klass_info["model"]
TypeError: 'NoneType' object is not subscriptable

In your set_custom_manager add the following and check, if you still get the error manager.contribute_to_class(model_class, manager_name) — Khan Asfi Reza
– Khan Asfi Reza, Commented Sep 14, 2024 at 22:28
Exactly what are you trying to do? Using a method to monkey patch is probably not a good idea. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Sep 15, 2024 at 17:17

Khan Asfi Reza · Accepted Answer · 2024-09-15 10:46:25Z

1

In your set_custom_manager add the following and check, if you still get the error

manager.contribute_to_class(model_class, manager_name)

answered Sep 15, 2024 at 10:46

Khan Asfi Reza

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