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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
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commit 6fa30faa797fd826b6c91aedd13a430430f11d35 1 parent 6828821
Malcolm Tredinnick malcolmt authored
Showing with 53 additions and 0 deletions.
  1. +2 −0  docs/ref/models/querysets.txt
  2. +51 −0 docs/topics/db/aggregation.txt
2  docs/ref/models/querysets.txt
View
@@ -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()``
~~~~~~~~~~~~~~
51 docs/topics/db/aggregation.txt
View
@@ -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() <querysets-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
-----------------------
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