#27499: Pickling a QuerySet evaluates the querysets given to Prefetch in
prefetch_related
-------------------------------------+-------------------------------------
Reporter: Adam | Owner: nobody
Chainz |
Type: | Status: new
Uncategorized |
Component: | Version: 1.10
Uncategorized |
Severity: Normal | Keywords: prefetch_related
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
Wrote a minimal test case already for `tests/queryset_pickle/tests.py`:
{{{
def test_pickle_prefetch_related_queryset_not_evaluated(self):
Group.objects.create(name='foo')
groups = Group.objects.prefetch_related(
models.Prefetch('event_set',
queryset=Event.objects.order_by('id'))
)
list(groups) # evaluate QS
with self.assertNumQueries(0):
pickle.dumps(groups)
}}}
At current doing `pickle.dumps` evaluates `Event.objects.order_by('id')`
which is obviously a bad thing, especially if `Event` is a large table!
Found this as a kind of regression when upgrading 1.9 -> 1.10 because the
`prefetch_related` refactoring seems to be keeping the `Prefetch` objects
around more, but afaict it has always existed.
--
Ticket URL: <https://code.djangoproject.com/ticket/27499>
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