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Refs #14645 -- Documented bug with exclude() and multi-value relations

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David Seddon 2015-01-12 16:11:11 +00:00 • committed by Tim Graham
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39
docs/topics/db/queries.txt
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@ -546,8 +546,7 @@ find entries linked to tags called *"music"* and *"bands"* or we might want an
entry that contains a tag with a name of *"music"* and a status of *"public"*.
To handle both of these situations, Django has a consistent way of processing
:meth:`~django.db.models.query.QuerySet.filter` and
:meth:`~django.db.models.query.QuerySet.exclude` calls. Everything inside a
:meth:`~django.db.models.query.QuerySet.filter` calls. Everything inside a
single :meth:`~django.db.models.query.QuerySet.filter` call is applied
simultaneously to filter out items matching all those requirements. Successive
:meth:`~django.db.models.query.QuerySet.filter` calls further restrict the set
@ -581,14 +580,34 @@ that were published in 2008. The entries selected by the second filter may or
may not be the same as the entries in the first filter. We are filtering the
``Blog`` items with each filter statement, not the ``Entry`` items.
All of this behavior also applies to
:meth:`~django.db.models.query.QuerySet.exclude`: all the conditions in a
single :meth:`~django.db.models.query.QuerySet.exclude` statement apply to a
single instance (if those conditions are talking about the same multi-valued
relation). Conditions in subsequent
:meth:`~django.db.models.query.QuerySet.filter` or
:meth:`~django.db.models.query.QuerySet.exclude` calls that refer to the same
relation may end up filtering on different linked objects.
.. note::
The behavior of :meth:`~django.db.models.query.QuerySet.filter` for queries
that span multi-value relationships, as described above, is not implemented
equivalently for :meth:`~django.db.models.query.QuerySet.exclude`. Instead,
the conditions in a single :meth:`~django.db.models.query.QuerySet.exclude`
call will not necessarily refer to the same item.
For example, the following query would exclude blogs that contain *both*
entries with *"Lennon"* in the headline *and* entries published in 2008::
Blog.objects.exclude(
entry__headline__contains='Lennon',
entry__pub_date__year=2008,
)
However, unlike the behavior when using
:meth:`~django.db.models.query.QuerySet.filter`, this will not limit blogs
based on entries that satisfying both conditions. In order to do that, i.e.
to select all blogs that do not contain entries published with *"Lennon"*
that were published in 2008, you need to make two queries::
Blog.objects.exclude(
entry=Entry.objects.filter(
headline__contains='Lennon',
pub_date__year=2008,
),
)
.. _using-f-expressions-in-filters: