maximum recursion error when caching querysets on models

[zbyte64]

The following test case causes a runtime exception against django 1.2 beta 1:

from django.test import TestCase
from django.db import models

class TestModel(models.Model):
    def cachedmethod(self):
        if not hasattr(self, '_cachedmethod'):
            self._cachedmethod = RelatedModel.objects.all().filter(test_model=self)
        return self._cachedmethod

class RelatedModel(models.Model):
    test_model = models.ForeignKey(TestModel)
    category = models.CharField(max_length=50)

class dummy(object):
    def __init__(self, parent):
        self.parent = parent
    
    def cachedmethod(self):    
        if not hasattr(self, '_cachedmethod'):
            self._cachedmethod = RelatedModel.objects.all().filter(test_model=self.parent)
        return self._cachedmethod

class BreakMeTest(TestCase):
    def test_caching(self):
        parent = TestModel()
        parent.save()
        related = RelatedModel(category='foo', test_model=parent)
        related.save()
        
        adummy = dummy(parent)
        adummy.cachedmethod()
        adummy.cachedmethod().filter(category='foo') #this doesn't break it
        parent.cachedmethod()
        parent.cachedmethod().filter(category='foo') #this breaks it

[zbyte64]

Stack trace of exception

[elachuni]

django.db.models.sql.Query defines a custom __deepcopy__ that calls self.clone(), disregarding the provided memo dict.

So, if the query contains a reference to a model instance that in turn contains a reference to a reference (as in this case), deepcopy will end up in an endless loop.

The model itself could work around this problem by providing its own __deepcopy__ implementation, that just didn't include the query. For the example in the bugreport:

class TestModel(models.Model):
    def __deepcopy__(self, memo):
        return TestModel(id=self.id)
    ....

[elachuni]

The attached patch adds an optional argument 'memo' to clone, to be able to correctly implement deepcopy.

Replying to elachuni:

So, if the query contains a reference to a model instance that in turn contains a reference to a reference (...)

Should have been a reference to a query here.

[elachuni]

Patch with a brief doctest

[Russell Keith-Magee]

(In [12700]) Fixed #12876 -- Corrected a problem with recursive relations under deepcopy. Thanks to elachuni for the patch.

[Russell Keith-Magee]

(In [12702]) [1.1.X] Fixed #12876 -- Corrected a problem with recursive relations under deepcopy. Thanks to elachuni for the patch.

Backport of r12700 from trunk.

[Jacob]

Milestone 1.2 deleted