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Code Issues 32 Releases Wiki Activity

Fixed #24190 -- Clarified len(queryset)

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Collin Anderson 2015-01-20 10:20:02 -05:00 committed by Tim Graham
parent 0386b97706
commit ee23e03637
1 changed files with 11 additions and 8 deletions
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19
docs/ref/models/querysets.txt
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@ -57,11 +57,10 @@ You can evaluate a ``QuerySet`` in the following ways:
* **len().** A ``QuerySet`` is evaluated when you call ``len()`` on it. * **len().** A ``QuerySet`` is evaluated when you call ``len()`` on it.
This, as you might expect, returns the length of the result list. This, as you might expect, returns the length of the result list.
Note: *Don't* use ``len()`` on ``QuerySet``\s if all you want to do is Note: If you only need to determine the number of records in the set (and
determine the number of records in the set. It's much more efficient to don't need the actual objects), it's much more efficient to handle a count
handle a count at the database level, using SQL's ``SELECT COUNT(*)``, at the database level using SQL's ``SELECT COUNT(*)``. Django provides a
and Django provides a ``count()`` method for precisely this reason. See :meth:`~QuerySet.count` method for precisely this reason.
``count()`` below.
* **list().** Force evaluation of a ``QuerySet`` by calling ``list()`` on * **list().** Force evaluation of a ``QuerySet`` by calling ``list()`` on
it. For example:: it. For example::
@ -76,9 +75,8 @@ You can evaluate a ``QuerySet`` in the following ways:
if Entry.objects.filter(headline="Test"): if Entry.objects.filter(headline="Test"):
print("There is at least one Entry with the headline Test") print("There is at least one Entry with the headline Test")
Note: *Don't* use this if all you want to do is determine if at least one Note: If you only want to determine if at least one result exists (and don't
result exists, and don't need the actual objects. It's more efficient to need the actual objects), it's more efficient to use :meth:`~QuerySet.exists`.
use :meth:`~QuerySet.exists` (see below).
.. _pickling QuerySets: .. _pickling QuerySets:
@ -1805,6 +1803,11 @@ Depending on which database you're using (e.g. PostgreSQL vs. MySQL),
is an underlying implementation quirk that shouldn't pose any real-world is an underlying implementation quirk that shouldn't pose any real-world
problems. problems.
Note that if you want the number of items in a ``QuerySet`` and are also
retrieving model instances from it (for example, by iterating over it), it's
probably more efficient to use ``len(queryset)`` which won't cause an extra
database query like ``count()`` would.
in_bulk in_bulk
~~~~~~~ ~~~~~~~