Prefetc_related on a field with tracker fails

[mahsa-lotfi92]

Problem

Running prefetch_related on a column which has a tracker will be failed with this error:

ValueError: "prefetch_related_field_name" does not resolve to an item that supports prefetching - this is an invalid parameter to prefetch_related().

Environment

• Django Model Utils version: 4.0.0
• Django version: 2.2
• Python version: 3.8
• Other libraries used, if any: nothing.

Code examples

Given this code in models.py

from django.db import models
from model_utils import FieldTracker
class A(models.Model):
    name = models.CharField(max_length=10)


class B(models.Model):
    name = models.CharField(max_length=10)


class C(models.Model):
    a = models.ForeignKey(A, on_delete=models.CASCADE)
    b = models.ForeignKey(B, on_delete=models.CASCADE)
    tracker = FieldTracker(fields=('b',))

This is the test case to reproduce the error:

from django.test import TestCase
from models import *


class TestTracker(TestCase):
    def test_tracker(self):
        a = A.objects.create(name='something')
        b = B.objects.create(name='something')
        C.objects.create(a=a, b=b)
        print(A.objects.prefetch_related('c_set__b').all())

and this is the error:

ValueError: 'c_set__b' does not resolve to an item that supports prefetching - this is an invalid parameter to prefetch_related().

I debugged and found that when running the executing prefetch in Django, rel_obj_descriptor is tracker.DescriptorWrapper instead of ForwardManyToOneDescriptor which does not have the mehtod get_prefetch_queryset

[IBestuzhev]

As a workaround, I managed to have Tracker + Prefetch working together by adding this lines:

C.b.get_prefetch_queryset = C.b.descriptor.get_prefetch_queryset
C.b.is_cached = C.b.descriptor.is_cached

But I guess the correct fix is to alter DescriptorWrapper. It should include something like

class DescriptorWrapper:
    # ...
    def __getattr__(self, attr):
        return getattr(self.descriptor, attr)