django/docs/ref/models/queries.txt

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=====================
Query-related classes
=====================
.. currentmodule:: django.db.models
This document provides reference material for query-related tools not
documented elsewhere.
``Q()`` objects
===============
.. class:: Q
A ``Q()`` object, like an :class:`~django.db.models.F` object, encapsulates a
SQL expression in a Python object that can be used in database-related
operations.
In general, ``Q() objects`` make it possible to define and reuse conditions.
This permits the :ref:`construction of complex database queries
<complex-lookups-with-q>` using ``|`` (``OR``) and ``&`` (``AND``) operators;
in particular, it is not otherwise possible to use ``OR`` in ``QuerySets``.
``Prefetch()`` objects
======================
.. versionadded:: 1.7
.. class:: Prefetch(lookup, queryset=None, to_attr=None)
The ``Prefetch()`` object can be used to control the operation of
:meth:`~django.db.models.query.QuerySet.prefetch_related()`.
The ``lookup`` argument describes the relations to follow and works the same
as the string based lookups passed to
:meth:`~django.db.models.query.QuerySet.prefetch_related()`.
The ``queryset`` argument supplies a base ``QuerySet`` for the given lookup.
This is useful to further filter down the prefetch operation, or to call
:meth:`~django.db.models.query.QuerySet.select_related()` from the prefetched
relation, hence reducing the number of queries even further.
The ``to_attr`` argument sets the result of the prefetch operation to a custom
attribute.
.. note::
When using ``to_attr`` the prefetched result is stored in a list.
This can provide a significant speed improvement over traditional
``prefetch_related`` calls which store the cached result within a
``QuerySet`` instance.