#17271: QuerySet.none() doesn't work well with subclasses of QuerySet
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Reporter: andreypopp | Owner: nobody
Type: Bug | Status: new
Component: Database layer (models, ORM) | Version: SVN
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 1
Easy pickings: 1 | UI/UX: 0
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I use `QuerySet` subclassing to allow building complex composable queries
out of smaller ones:
{{{
class MyQuerySet(QuerySet):
def allowed(self, user):
return self.filter(permissions__in=user.permissions)
def have_feature(feature_name):
return self.filter(feature__name=feature_name)
...
objs = Obj.objects.allowed(user).have_feature("somefeature")
}}}
Sometimes I want to return `EmptyQuerySet` by calling `none()` to provide
"fast-path" for some conditions, but unfortunately `EmptyQuerySet` doesn't
have my custom methods, so call chain breaks after returning it:
{{{
class MyQuerySet(QuerySet):
def allowed(self, user):
if user.is_anonymous:
return self.none()
return self.filter(permissions__in=user.permissions)
def have_feature(feature_name):
return self.filter(feature__name=feature_name)
...
objs = Obj.objects.allowed(user).have_feature("some feature")
# AttributeError: EmptyQuerySet doesn't have have_feature attribute ...
}}}
I've attached a patch which instruments `EmptyQuerySet` subclass for each
particular `QuerySet` implementation and returns its instance as a result
of `none()` method call. I've also refactored code in `Manager.none()` to
reuse existing code in `QuerySet.none()` and to parametrize `Manager()`
constructor over `QuerySet` implementation to use.
--
Ticket URL: <https://code.djangoproject.com/ticket/17271>
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