#34889: Broken fallback for prefetchers that only implement
get_prefetch_queryset
-------------------------------------+-------------------------------------
Reporter: Matt | Owner: nobody
Westcott |
Type: Bug | Status: new
Component: Database | Version: 5.0
layer (models, ORM) |
Severity: Release | Keywords:
blocker |
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
In #33651, the `get_prefetch_queryset()` method of related managers and
descriptors is deprecated in favour of `get_prefetch_querysets()`. Based
on the deprecation warnings, it's clear that subclasses implementing only
`get_prefetch_queryset()` are intended to continue working until this is
removed outright in 6.0. However, the fallback code in
`prefetch_one_level` calls it with an invalid signature:
https://github.com/django/django/blob/4e790271e3e65c9ad037b347a34fa95e11982228/django/db/models/query.py#L2535-L2562
Here the `get_prefetch_queryset` method is passed
`lookup.get_current_querysets(level)` - which returns a list of querysets
- as the `queryset` argument, which expects a single queryset. Changing
this to `lookup.get_current_queryset(level)` fixes the problem (although
it also doubles up the deprecation warnings, which may not be ideal).
Unfortunately I'm slightly stumped over how I might create a regression
test for this - presumably this would involve writing a minimal related
manager class that implements `get_current_queryset` but not
`get_current_querysets`, but since this is an internal API it's hard to
know what's considered minimal while still being a legal implementation.
If it's any help, I encountered this as a regression to the `django-
modelcluster` package: https://github.com/wagtail/django-
modelcluster/actions/runs/6407673311/job/17394971115
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Ticket URL: <https://code.djangoproject.com/ticket/34889>
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