我如何以及为什么不能覆盖 django 上的相关管理器方法？

Question

I have this manager:

class ConfigValueManager(models.Manager):

    def get(self, key):
        config_value = self.filter(key=key).first()
        if config_value:
            type_caster = locate(config_value.type)
            return type_caster(config_value.value)
        return config_value

    def set(self, key, value):
        self.filter(key=key).update(value=value)

    def set2(self, key, value):
        qs = self.filter(key=key)
        if qs:
            qs.update(value=value, type=type(value).__name__, company=self.instance)
        else:
            self.create(key=key, value=value, type=type(value).__name__, company=self.instance)

the problem is that I can't overwrite set . The method is still coming from the parent, even though I've created set on the child. Funny thing is that get and set2 are fine. Even add which isn't in my example can't be overridden.

My question is how can I overwrite set and why this happens?

Answer 1

I add some details on why it's not easily possible, because I struggled on the same issue. set , like add or create , are overridden in the dynamically created RelatedManager , as we can see in the django source code . This RelatedManager actually uses our manager as a super class that's why your get and set2 methods can used, but it does not help for overridden methods.

This manager is created in the ReverseManyToOneDescriptor.related_manager_cls cached property. In the example on your github snippet, Company.config_values is an instance of this ReverseManyToOneDescriptor .

I'll show an example on how to override the set method, by making some assumptions on your code, because it misses some definitions (like the Company model, the ForeignKey field inside FooConfigValue .)

I don't advise to use it, as it's absolutely not robust against django changes, and I didn't do any test, it just serves as a proof on how RelatedManager instances are created Add this at the end of the example code and it should work

def modify_related_manager_set(model_cls):
    # model_cls = Company here, and config_values is the related field name
    reverse_descriptor = model_cls.config_values
    base_set = reverse_descriptor.related_manager_cls.set

    def custom_set(*args, **kwargs):
        print("in my custom set")
        return base_set(*args, **kwargs)

    reverse_descriptor.related_manager_cls.set = custom_set
# do this call after all the models have been created
# e.g. after defining FooConfigValue
 modify_related_manager_set(Company) 

And you should now see the in my custom set being printed.

I know this doesn't help much, but at least it helped understand how related fields work

Answer 2

models.py

from django.db import models
from django.db.models.query import QuerySet


class PersonQuerySet(QuerySet):
    def set(self, slug, **kwargs):
        return self.filter(slug=slug).update(**kwargs)


class Person(models.Model):
    name = models.CharField(max_length=100, null=True)
    slug = models.CharField(max_length=10, null=True)

    objects = PersonQuerySet.as_manager()

tests.py

from django.test import TestCase
from core.models import Person

class TestSet(TestCase):
    def test_just_update_records_with_the_same_slug(self):
        Person.objects.create(slug='batman', name='John')
        Person.objects.create(slug='batman', name='Connor')
        Person.objects.create(slug='bruce', name='Ill be back')

        Person.objects.set('batman', name='###')

        expected_value = 2
        result = Person.objects.filter(name='###').count()

        self.assertEqual(result, expected_value)

github example

https://github.com/luivilella/django-test-manager