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Code Issues 25 Releases Wiki Activity

Doc'd the need to remove default ordering on Subquery aggregates.

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Tomer Chachamu 2017-06-20 19:02:43 +01:00 committed by Tim Graham
parent 12812f6b2d
commit 62917cee5a
1 changed files with 6 additions and 5 deletions
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11
docs/ref/models/expressions.txt
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@ -601,15 +601,16 @@ Assuming both models have a ``length`` field, to find posts where the post
length is greater than the total length of all combined comments::
>>> from django.db.models import OuterRef, Subquery, Sum
>>> comments = Comment.objects.filter(post=OuterRef('pk')).values('post')
>>> comments = Comment.objects.filter(post=OuterRef('pk')).order_by().values('post')
>>> total_comments = comments.annotate(total=Sum('length')).values('total')
>>> Post.objects.filter(length__gt=Subquery(total_comments))
The initial ``filter(...)`` limits the subquery to the relevant parameters.
``values('post')`` aggregates comments by ``Post``. Finally, ``annotate(...)``
performs the aggregation. The order in which these queryset methods are applied
is important. In this case, since the subquery must be limited to a single
column, ``values('total')`` is required.
``order_by()`` removes the default :attr:`~django.db.models.Options.ordering`
(if any) on the ``Comment`` model. ``values('post')`` aggregates comments by
``Post``. Finally, ``annotate(...)`` performs the aggregation. The order in
which these queryset methods are applied is important. In this case, since the
subquery must be limited to a single column, ``values('total')`` is required.
This is the only way to perform an aggregation within a ``Subquery``, as
using :meth:`~.QuerySet.aggregate` attempts to evaluate the queryset (and if