#36426: prefetch_related_objects() is doc'd to accept iterables but only accepts
sequences
-------------------------------------+-------------------------------------
Reporter: Jacob Walls | Owner: (none)
Type: Bug | Status: new
Component: Database layer | Version: 5.2
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 1 | UI/UX: 0
-------------------------------------+-------------------------------------
Description changed by Jacob Walls:
Old description:
> The docs say `prefetch_related_objects()`
> [https://docs.djangoproject.com/en/5.2/ref/models/querysets/#django.db.models.prefetch_related_objects
> accepts iterables], but it only accepts sequences.
>
> {{{#!py
> 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 [https://github.com/typeddjango/django-
> stubs/blob/5bb841674610e88a162475b7ecf2e950a92c23a8/django-
> stubs/db/models/query.pyi#L247 types] the argument as `Model[Iterable]`.
New description:
The docs say `prefetch_related_objects()`
[https://docs.djangoproject.com/en/5.2/ref/models/querysets/#django.db.models.prefetch_related_objects
accepts iterables], but it only accepts sequences.
{{{#!py
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 [https://github.com/typeddjango/django-
stubs/blob/5bb841674610e88a162475b7ecf2e950a92c23a8/django-
stubs/db/models/query.pyi#L247 types] the argument as `Iterable[_Model]`.
--
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Ticket URL: <https://code.djangoproject.com/ticket/36426#comment:1>
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