#34791: Issue when using Prefetch objects in prefetch_related
-------------------------------------+-------------------------------------
Reporter: | Owner: nobody
MaxDude132 |
Type: Bug | Status: new
Component: Database | Version: 4.2
layer (models, ORM) | Keywords: prefetch,
Severity: Normal | prefetch_related
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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There seems to be an issue when using a Prefetch object to fetch something
that has already been fetched. Here is an example: so let's say I have a
Pizza and some Toppings. Now I want to get all the toppings, but for some
reason I also want to separately fetch only the toppings that are out of
stock. I could do something like:
{{{
queryset = Pizza.objects.all().prefetch_related(
'toppings',
Prefetch('toppings',
queryset=Topping.objects.filter(is_in_stock=False),
to_attr='out_of_stock_toppings'),
)
}}}
This looks good, but if we run it, it will fail, saying
out_of_stock_toppings is not an attribute of Pizza. However, if I were to
do it like this instead:
{{{
queryset = Pizza.objects.all().prefetch_related(
Prefetch('toppings',
queryset=Topping.objects.filter(is_in_stock=False),
to_attr='out_of_stock_toppings'),
'toppings',
)
}}}
then all works fine. Looking at the code, this seems to be because the
name used by a field to validate the cache is not the name used to store
the data on the model, but rather simply the name of the field, so it
collides. I have not tested it, but I think in the second example, the
data returned will actually be the filtered data, not the full expected
queryset. Note that this is my first time reading that part of the code,
so there could be things I missed.
Now this is a bit of a nonsensical example, but when using rest_framework
with serializers, this type of situation could come up, where one
serializer needs it formatted a certain way and this issue could arise (it
has for me).
I am not sure what the best way to fix this would be, but I feel like
setting a to_attr should make the cache take that new field name into
account instead of the field name.
--
Ticket URL: <https://code.djangoproject.com/ticket/34791>
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