From 6fa30faa797fd826b6c91aedd13a430430f11d35 Mon Sep 17 00:00:00 2001 From: Malcolm Tredinnick Date: Wed, 25 Mar 2009 09:05:38 +0000 Subject: [PATCH] Fixed #10574 -- Documented interaction between annotations and order_by. In the future, I'd like to fix this properly, but the current behavior has the advantage of being consistent across the board (and changing it everywhere is backwards-incompatible with documented functionality). git-svn-id: http://code.djangoproject.com/svn/django/trunk@10172 bcc190cf-cafb-0310-a4f2-bffc1f526a37 --- docs/ref/models/querysets.txt | 2 ++ docs/topics/db/aggregation.txt | 51 ++++++++++++++++++++++++++++++++++ 2 files changed, 53 insertions(+) diff --git a/docs/ref/models/querysets.txt b/docs/ref/models/querysets.txt index 034ed6640b..847cebf1e9 100644 --- a/docs/ref/models/querysets.txt +++ b/docs/ref/models/querysets.txt @@ -296,6 +296,8 @@ a model which defines a default ordering, or when using ordering was undefined prior to calling ``reverse()``, and will remain undefined afterward). +.. _querysets-distinct: + ``distinct()`` ~~~~~~~~~~~~~~ diff --git a/docs/topics/db/aggregation.txt b/docs/topics/db/aggregation.txt index 14579598b9..9f7ba6fa22 100644 --- a/docs/topics/db/aggregation.txt +++ b/docs/topics/db/aggregation.txt @@ -315,6 +315,57 @@ will be automatically added to the result set. However, if the ``values()`` clause is applied after the ``annotate()`` clause, you need to explicitly include the aggregate column. +Interaction with default ordering or ``order_by()`` +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Fields that are mentioned in the ``order_by()`` part of a queryset (or which +are used in the default ordering on a model) are used when selecting the +output data, even if they are not otherwise specified in the ``values()`` +call. These extra fields are used to group "like" results together and they +can make otherwise identical result rows appear to be separate. This shows up, +particularly, when counting things. + +By way of example, suppose you have a model like this:: + + class Item(models.Model): + name = models.CharField(max_length=10) + data = models.IntegerField() + + class Meta: + ordering = ["name"] + +The important part here is the default ordering on the ``name`` field. If you +want to count how many times each distinct ``data`` value appears, you might +try this:: + + # Warning: not quite correct! + Item.objects.values("data").annotate(Count("id")) + +...which will group the ``Item`` objects by their common ``data`` values and +then count the number of ``id`` values in each group. Except that it won't +quite work. The default ordering by ``name`` will also play a part in the +grouping, so this query will group by distinct ``(data, name)`` pairs, which +isn't what you want. Instead, you should construct this queryset:: + + Item.objects.values("data").annotate(Count("id")).order_by() + +...clearing any ordering in the query. You could also order by, say, ``data`` +without any harmful effects, since that is already playing a role in the +query. + +This behavior is the same as that noted in the queryset documentation for +:ref:`distinct() ` and the general rule is the same: +normally you won't want extra columns playing a part in the result, so clear +out the ordering, or at least make sure it's restricted only to those fields +you also select in a ``values()`` call. + +.. note:: + You might reasonably ask why Django doesn't remove the extraneous columns + for you. The main reason is consistency with ``distinct()`` and other + places: Django **never** removes ordering constraints that you have + specified (and we can't change those other methods' behavior, as that + would violate our :ref:`misc-api-stability` policy). + Aggregating annotations -----------------------