#37027: refresh_from_db() with from_queryset + prefetch does not persist result
in
instance
-------------------------------------+-------------------------------------
Reporter: Hugo Maingonnat | Owner: (none)
Type: Bug | Status: new
Component: Database layer | Version: 5.2
(models, ORM) |
Severity: Normal | Resolution:
Keywords: refresh_from_db, | Triage Stage:
prefetch | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by Simon Charette):
There's is little value in using `prefetch_related` when dealing with a
single object as this method is meant to be used to prevent N+1 query
issues but in this case `N=1` so the number of queries will be the same.
In other words, performing `foo.bar_set.all()` will issue the same query
as what a prefetching would have done.
When we added `refresh_from_db(from_queryset)` support (#28344) we knew it
would open the door to a large surface area between `Model` and `QuerySet`
APIs. Given the original intent was to support `for_update` and friends
I'm not convinced we should add support for `prefetch_related` and
certainly not for `Prefetch(to_attr)` as it doesn't follow the
`refresh_from_db(fields)` convention.
Support for `select_related` came for free as it's a normal attribute
lookup on the retrieved model instance and there are real benefits to
using it (reducing the number of queries). In the case of
`prefetch_related` it requires extra retrieval and invalidation logic of
many-to-many relationships and results in the same number of queries.
--
Ticket URL: <https://code.djangoproject.com/ticket/37027#comment:1>
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