Unless someone objects to adjusting Model.__copy__(self) to deal with self._state.fields_cache I'd say we should create a ticket about it.
Would you be interested in creating the ticket and possibly submitting a patch Gert? Simon Le mardi 4 août 2020 à 21:23:47 UTC-4, Alex Hill a écrit : > I reckon stick with your first instinct SImon. > > I don't think using copy.copy needs to be an explicitly documented > pattern. It's a heavily-used part of the standard library, and the objects > Django provides should work with it as well as they can. The behaviour is > surprising and buggy at face value: changing the value of an attribute on > one object should not change the value of that attribute on another! > > Alex > > On Wed, Aug 5, 2020 at 7:43 AM charettes <chare...@gmail.com> wrote: > >> It was likely overlooked by the patch. >> >> Looks like Model.__copy__ should make sure to make a deep-copy of >> self._state now that fields are cached in self._state.fields_cache. >> >> Using copy.deepcopy will circumvent the issue but I feel like copy.copy >> is common enough pattern that we should maintain compatibility for it. >> After all, the fact the _state_ of the model is now stored in >> _state.fields_cache is no more than an implementation detail and it would >> be normal to expect shallow copying to work in this case. On the other hand >> the fact this has been happening since Django 2.0 and has just been >> reported and that shallow copies won't work correctly for prefetched >> relationships are both non-negligible argument for a wontfix since this >> pattern was never explicitly documented. >> >> There is a precedent for a similar undocumented pattern though [0]. >> >> Thoughts? >> >> Simon >> >> [0] >> https://github.com/django/django/commit/dee5896b55385d2c66d8c1b8386604868f7fc6b4 >> >> Le mardi 4 août 2020 à 10:48:43 UTC-4, gertb...@gmail.com a écrit : >> >>> Hi Simon, >>> >>> I think the commit is bfb746f983aa741afa3709794e70f1e0ab6040b5 "Refs >>> #16043 -- Refactored internal fields value cache". >>> >>> Cheers >>> >>> On Tue, 4 Aug 2020 at 15:52, charettes <chare...@gmail.com> wrote: >>> >>>> Hello Gert, that seems a bit surprising to me and was likely not a >>>> desired change. >>>> >>>> Could you bisect the exact commit that caused the regression[0]? That >>>> would certainly help determining the action to take here. >>>> >>>> Thanks, >>>> Simon >>>> >>>> [0] >>>> https://docs.djangoproject.com/en/3.1/internals/contributing/triaging-tickets/#bisecting-a-regression >>>> >>>> Le mardi 4 août 2020 à 08:10:00 UTC-4, gertb...@gmail.com a écrit : >>>> >>>>> (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 >>>>> >>>> -- >>>> You received this message because you are subscribed to the Google >>>> Groups "Django developers (Contributions to Django itself)" group. >>>> To unsubscribe from this group and stop receiving emails from it, send >>>> an email to django-develop...@googlegroups.com. >>>> To view this discussion on the web visit >>>> https://groups.google.com/d/msgid/django-developers/c284e35e-ae02-44e5-b44d-4994eff9b248n%40googlegroups.com >>>> >>>> <https://groups.google.com/d/msgid/django-developers/c284e35e-ae02-44e5-b44d-4994eff9b248n%40googlegroups.com?utm_medium=email&utm_source=footer> >>>> . >>>> >>> -- >> You received this message because you are subscribed to the Google Groups >> "Django developers (Contributions to Django itself)" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to django-develop...@googlegroups.com. >> > To view this discussion on the web visit >> https://groups.google.com/d/msgid/django-developers/0b5d48d0-5594-40d6-9dce-80995f8635ecn%40googlegroups.com >> >> <https://groups.google.com/d/msgid/django-developers/0b5d48d0-5594-40d6-9dce-80995f8635ecn%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> > -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. 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