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[1.6.x] Documented a current limitation of multiple table annotation; refs #10060.

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Backport of 4b23a6c7a9232cc07ec95fe98be17efbd4449822 from master
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Benjamin Bach 2014-12-01 00:02:43 +01:00 committed by Tim Graham
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docs/topics/db/aggregation.txt
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@ -179,6 +179,17 @@ of the ``annotate()`` clause is a ``QuerySet``; this ``QuerySet`` can be
modified using any other ``QuerySet`` operation, including ``filter()``,
``order_by()``, or even additional calls to ``annotate()``.
.. admonition:: If in doubt, inspect the SQL query!
In order to understand what happens in your query, consider inspecting the
``query`` property of your ``QuerySet``.
For instance, combining multiple aggregations with ``annotate()`` will
yield the wrong results, as `multiple tables are the cross joined`_,
resulting in duplicate row aggregations.
.. _multiple tables are the cross joined: https://code.djangoproject.com/ticket/10060
Joins and aggregates
====================