Commit cc5c4ae3 authored by Tim Graham's avatar Tim Graham
Browse files

[1.9.x] Refs #25136 -- Fixed nonexistent field reference in aggregation topic guide. 

Thanks Ankush Thakur for the report and Simon for the review.

Backport of fe70f280 from master
parent 3f93f8d8

docs/topics/db/aggregation.txt

+7 −6
Original line number Diff line number Diff line
@@ -193,24 +193,25 @@ Combining multiple aggregations with ``annotate()`` will `yield the wrong 
results <https://code.djangoproject.com/ticket/10060>`_ because joins are used

instead of subqueries:



    >>> Book.objects.first().authors.count()

    >>> book = Book.objects.first()

    >>> book.authors.count()

    2

    >>> Book.objects.first().chapters.count()

    >>> book.store_set.count()

    3

    >>> q = Book.objects.annotate(Count('authors'), Count('chapters'))

    >>> q = Book.objects.annotate(Count('authors'), Count('store'))

    >>> q[0].authors__count

    6

    >>> q[0].chapters__count

    >>> q[0].store__count

    6



For most aggregates, there is no way to avoid this problem, however, the

:class:`~django.db.models.Count` aggregate has a ``distinct`` parameter that

may help:



    >>> q = Book.objects.annotate(Count('authors', distinct=True), Count('chapters', distinct=True))

    >>> q = Book.objects.annotate(Count('authors', distinct=True), Count('store', distinct=True))

    >>> q[0].authors__count

    2

    >>> q[0].chapters__count

    >>> q[0].store__count

    3



.. admonition:: If in doubt, inspect the SQL query!