Opened 4 months ago

Last modified 10 days ago

#36426 closed Cleanup/optimization

prefetch_related_objects() is doc'd to accept iterables but only accepts sequences — at Initial Version

Reported by: Jacob Walls Owned by:
Component: Database layer (models, ORM) Version: dev
Severity: Normal Keywords:
Cc: Triage Stage: Ready for checkin
Has patch: yes Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: yes UI/UX: no

Description

The docs say prefetch_related_objects() accepts iterables, but it only accepts sequences.

In [1]: from django.db.models import prefetch_related_objects

In [2]: objs = set(Graph.objects.all())

In [3]: prefetch_related_objects(objs, "node_set")
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[3], line 1
----> 1 prefetch_related_objects(objs, "node_set")

File ~/py313/lib/python3.13/site-packages/django/db/models/query.py:2361, in prefetch_related_objects(model_instances, *related_lookups)
   2355     break
   2357 # Descend down tree
   2358 
   2359 # We assume that objects retrieved are homogeneous (which is the premise
   2360 # of prefetch_related), so what applies to first object applies to all.
-> 2361 first_obj = obj_list[0]
   2362 to_attr = lookup.get_current_to_attr(level)[0]
   2363 prefetcher, descriptor, attr_found, is_fetched = get_prefetcher(
   2364     first_obj, through_attr, to_attr
   2365 )

TypeError: 'set' object is not subscriptable

We could adjust the docs, but at a glance we could probably instead adjust the implementation to just next() instead of [0] and accept iterables as documented.

django-stubs types the argument as Model[Iterable].

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