(initially posted to
https://forum.djangoproject.com/t/fk-field-caching-behavior-change-between-1-11-x-and-2-x/3151
on 26 June but this mailing list might be more appropriate)
Hi
Whilst upgrading a codebase from 1.11.x to 2.0/2.2 I noticed a weird change
in behavior of FK fields when copying model instances.
At the bottom of the post there is a testcase that succeeds on 1.11.x and
fails on 2.x
I think the commit that changed the behavior is
bfb746f983aa741afa3709794e70f1e0ab6040b5
So my question is two fold:
1. Is the behavior in >=2.0 correct? It seems quite unexpected.
2. What is the recommended way to clone a model instance? To date we
have been using copy() in a similar fashion to the test without issue.
deepcopy seems to work fine in >=2.0 but we haven’t done too much
testing yet.
Test (placed in tests/model_fields/test_field_caching_change.py):
---------------------code--------------------------
import copy
from django.test import TestCase
from .models import Bar, Foo
class ForeignKeyCachingBehaviorTest(TestCase):
def test_copy(self):
foo1 = Foo.objects.create(a='foo1', d=1)
foo2 = Foo.objects.create(a='foo2', d=2)
bar1 = Bar.objects.create(a=foo1, b='bar1')
bar2 = copy.copy(bar1)
bar2.pk = None
bar2.a = foo2
# bar2 points to foo2
self.assertEqual(bar2.a, foo2)
self.assertEqual(bar2.a.id, bar2.a_id)
# bar1 is unchanged and must still point to foo1
# These fail on Django >= 2.0
self.assertEqual(bar1.a, foo1)
self.assertEqual(bar1.a.id, bar1.a_id)
---------------------code--------------------------
and executed that via:
python3.6 tests/runtests.py --parallel 1 model_fields
Regards
Gert Burger
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