mirror of
https://github.com/django/django.git
synced 2024-12-29 12:36:08 +00:00
52 lines
1.7 KiB
Plaintext
52 lines
1.7 KiB
Plaintext
=====================
|
|
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.
|