#25693: Data loss if a ManyToManyField is shadowed by Prefetch
-------------------------------------+-------------------------------------
Reporter: Ian-Foote | Owner: Ian-Foote
Type: Bug | Status: assigned
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Description changed by Ian-Foote:
Old description:
> With two models:
>
> {{{#!python
> class Book(Model):
> name = CharField(max_length=100)
>
> class Author(Model):
> books = ManyToManyField(Book)
> }}}
>
> it is possible to create a prefetch query that loses data from
> Author.books:
>
> {{{#!python
> poems = Book.objects.filter(name='Poems')
> Author.objects.prefetch_related(
> Prefetch('books', queryset=poems, to_attr='books'),
> )
> }}}
>
> When this queryset is evaluated, each Author's books is overridden by the
> poems queryset.
New description:
With two models:
{{{#!python
class Book(Model):
name = CharField(max_length=100)
class Author(Model):
books = ManyToManyField(Book)
}}}
it is possible to create a prefetch query that loses data from
{{{Author.books}}}:
{{{#!python
poems = Book.objects.filter(name='Poems')
Author.objects.prefetch_related(
Prefetch('books', queryset=poems, to_attr='books'),
)
}}}
When this queryset is evaluated, each Author's {{{books}}} is overridden
by the {{{poems}}} queryset.
--
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Ticket URL: <https://code.djangoproject.com/ticket/25693#comment:2>
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