Fixed #23878 -- Moved Query and Prefetch documentation

This commit is contained in:
Ng Zhi An 2015-01-11 20:02:33 +00:00 committed by Tim Graham
parent 58833f5197
commit 8f5d6c77b6
4 changed files with 55 additions and 53 deletions

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@ -67,7 +67,6 @@ manipulating the data of your Web application. Learn more about it below:
* **QuerySets:** * **QuerySets:**
:doc:`Executing queries <topics/db/queries>` | :doc:`Executing queries <topics/db/queries>` |
:doc:`QuerySet method reference <ref/models/querysets>` | :doc:`QuerySet method reference <ref/models/querysets>` |
:doc:`Query-related classes <ref/models/queries>` |
:doc:`Lookup expressions <ref/models/lookups>` :doc:`Lookup expressions <ref/models/lookups>`
* **Model instances:** * **Model instances:**

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@ -14,7 +14,6 @@ Model API reference. For introductory material, see :doc:`/topics/db/models`.
options options
instances instances
querysets querysets
queries
lookups lookups
expressions expressions
database-functions database-functions

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@ -1,51 +0,0 @@
=====================
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.

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@ -180,6 +180,9 @@ The lookup parameters (``**kwargs``) should be in the format described in
`Field lookups`_ below. Multiple parameters are joined via ``AND`` in the `Field lookups`_ below. Multiple parameters are joined via ``AND`` in the
underlying SQL statement. underlying SQL statement.
If you need to execute more complex queries (for example, queries with ``OR`` statements),
you can use :class:`Q objects <django.db.models.Q>`.
exclude exclude
~~~~~~~ ~~~~~~~
@ -215,6 +218,9 @@ In SQL terms, that evaluates to::
Note the second example is more restrictive. Note the second example is more restrictive.
If you need to execute more complex queries (for example, queries with ``OR`` statements),
you can use :class:`Q objects <django.db.models.Q>`.
annotate annotate
~~~~~~~~ ~~~~~~~~
@ -2843,3 +2849,52 @@ Variance
extension. extension.
.. _SQLite documentation: http://www.sqlite.org/contrib .. _SQLite documentation: http://www.sqlite.org/contrib
Query-related classes
=====================
This section 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.