Fixed #25136 -- Documented Count('X', distinct=True) in aggregate topic guide.

modified using any other ``QuerySet`` operation, including ``filter()``,

``order_by()``, or even additional calls to ``annotate()``.

Combining multiple aggregations

-------------------------------

Combining multiple aggregations with ``annotate()`` will `yield the wrong

results <https://code.djangoproject.com/ticket/10060>`_, as multiple tables are

cross joined. Due to the use of ``LEFT OUTER JOIN``, duplicate records will be

generated if some of the joined tables contain more records than the others:

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

    2

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

    3

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

    >>> q[0].authors__count

    6

    >>> q[0].chapters__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[0].authors__count

    2

    >>> q[0].chapters__count

    3

.. 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 cross joined`_,

    resulting in duplicate row aggregations.

.. _multiple tables are cross joined: https://code.djangoproject.com/ticket/10060

Joins and aggregates

====================