#30894: Reverse OneToOneField relation: `prefetch_related` uses `related_name`
while `select_related` uses `related_query_name`
-------------------------------------+-------------------------------------
Reporter: Adam | Owner: nobody
Sołtysik |
Type: Bug | Status: new
Component: Database | Version: 2.2
layer (models, ORM) |
Severity: Normal | Keywords:
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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The documentation
(https://docs.djangoproject.com/en/2.2/ref/models/querysets/#select-
related) states:
You can also refer to the reverse direction of a OneToOneField in the
list of fields passed to select_related — that is, you can traverse a
OneToOneField back to the object on which the field is defined. Instead of
specifying the field name, use the related_name for the field on the
related object.
But in fact `related_query_name` is used by `select_related` and
`related_name` is used by `prefetch_related` .
When we have the following models:
{{{
class A(Model):
pass
}}}
{{{
class B(Model):
a = OneToOneField('A', related_name='b_set', related_query_name='b')
}}}
then `a.select_related('b')` works and `a.prefetch_related('b')` does not:
{{{AttributeError: Cannot find 'b' on A object, 'b' is an invalid
parameter to prefetch_related()}}}
or if class `A` happens to have a property named `b`:
{{{ValueError: 'b' does not resolve to an item that supports prefetching -
this is an invalid parameter to prefetch_related().}}}
And vice versa, `a.prefetch_related('b_set')` works and
`a.select_related('b_set')` does not:
{{{django.core.exceptions.FieldError: Invalid field name(s) given in
select_related: 'b_set'. Choices are: b}}}
The documentation implies that both methods should use `related_name`, but
I think it would be more intuitive if both used `related_query_name`,
given that we can write lookups like in filter queries etc., and such a
change would probably be less backward-incompatible.
--
Ticket URL: <https://code.djangoproject.com/ticket/30894>
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