[1.8.x] Fixed #24190 -- Clarified len(queryset)

Backport of ee23e03637 from master
This commit is contained in:
Collin Anderson 2015-01-20 10:20:02 -05:00 committed by Tim Graham
parent 242c9538c8
commit 09e8985f25
1 changed files with 11 additions and 8 deletions

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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.
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
determine the number of records in the set. It's much more efficient to
handle a count at the database level, using SQL's ``SELECT COUNT(*)``,
and Django provides a ``count()`` method for precisely this reason. See
``count()`` below.
Note: If you only need to determine the number of records in the set (and
don't need the actual objects), it's much more efficient to handle a count
at the database level using SQL's ``SELECT COUNT(*)``. Django provides a
:meth:`~QuerySet.count` method for precisely this reason.
* **list().** Force evaluation of a ``QuerySet`` by calling ``list()`` on
it. For example::
@ -76,9 +75,8 @@ You can evaluate a ``QuerySet`` in the following ways:
if Entry.objects.filter(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
result exists, and don't need the actual objects. It's more efficient to
use :meth:`~QuerySet.exists` (see below).
Note: If you only want to determine if at least one result exists (and don't
need the actual objects), it's more efficient to use :meth:`~QuerySet.exists`.
.. _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
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
~~~~~~~