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161 lines
6.0 KiB
Plaintext
=====================
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Query-related classes
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=====================
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.. currentmodule:: django.db.models
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This document provides reference material for query-related tools not
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documented elsewhere.
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``F()`` expressions
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===================
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.. class:: F
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An ``F()`` object represents the value of a model field. It makes it possible
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to refer to model field values and perform database operations using them
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without actually having to pull them out of the database into Python memory.
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Instead, Django uses the ``F()`` object to generate a SQL expression that
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describes the required operation at the database level.
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This is easiest to understand through an example. Normally, one might do
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something like this::
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# Tintin filed a news story!
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reporter = Reporters.objects.get(name='Tintin')
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reporter.stories_filed += 1
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reporter.save()
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Here, we have pulled the value of ``reporter.stories_filed`` from the database
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into memory and manipulated it using familiar Python operators, and then saved
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the object back to the database. But instead we could also have done::
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from django.db.models import F
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reporter = Reporters.objects.get(name='Tintin')
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reporter.stories_filed = F('stories_filed') + 1
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reporter.save()
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Although ``reporter.stories_filed = F('stories_filed') + 1`` looks like a
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normal Python assignment of value to an instance attribute, in fact it's an SQL
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construct describing an operation on the database.
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When Django encounters an instance of ``F()``, it overrides the standard Python
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operators to create an encapsulated SQL expression; in this case, one which
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instructs the database to increment the database field represented by
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``reporter.stories_filed``.
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Whatever value is or was on ``reporter.stories_filed``, Python never gets to
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know about it - it is dealt with entirely by the database. All Python does,
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through Django's ``F()`` class, is create the SQL syntax to refer to the field
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and describe the operation.
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.. note::
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In order to access the new value that has been saved in this way, the object
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will need to be reloaded::
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reporter = Reporters.objects.get(pk=reporter.pk)
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As well as being used in operations on single instances as above, ``F()`` can
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be used on ``QuerySets`` of object instances, with ``update()``. This reduces
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the two queries we were using above - the ``get()`` and the
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:meth:`~Model.save()` - to just one::
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reporter = Reporters.objects.filter(name='Tintin')
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reporter.update(stories_filed=F('stories_filed') + 1)
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We can also use :meth:`~django.db.models.query.QuerySet.update()` to increment
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the field value on multiple objects - which could be very much faster than
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pulling them all into Python from the database, looping over them, incrementing
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the field value of each one, and saving each one back to the database::
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Reporter.objects.all().update(stories_filed=F('stories_filed') + 1)
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``F()`` therefore can offer performance advantages by:
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* getting the database, rather than Python, to do work
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* reducing the number of queries some operations require
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.. _avoiding-race-conditions-using-f:
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Avoiding race conditions using ``F()``
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--------------------------------------
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Another useful benefit of ``F()`` is that having the database - rather than
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Python - update a field's value avoids a *race condition*.
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If two Python threads execute the code in the first example above, one thread
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could retrieve, increment, and save a field's value after the other has
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retrieved it from the database. The value that the second thread saves will be
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based on the original value; the work of the first thread will simply be lost.
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If the database is responsible for updating the field, the process is more
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robust: it will only ever update the field based on the value of the field in
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the database when the :meth:`~Model.save()` or ``update()`` is executed, rather
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than based on its value when the instance was retrieved.
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Using ``F()`` in filters
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------------------------
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``F()`` is also very useful in ``QuerySet`` filters, where they make it
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possible to filter a set of objects against criteria based on their field
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values, rather than on Python values.
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This is documented in :ref:`using F() expressions in queries
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<using-f-expressions-in-filters>`
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Supported operations with ``F()``
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---------------------------------
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As well as addition, Django supports subtraction, multiplication, division,
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and modulo arithmetic with ``F()`` objects, using Python constants,
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variables, and even other ``F()`` objects.
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.. versionadded:: 1.7
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The power operator ``**`` is also supported.
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``Q()`` objects
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===============
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.. class:: Q
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A ``Q()`` object, like an :class:`~django.db.models.F` object, encapsulates a
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SQL expression in a Python object that can be used in database-related
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operations.
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In general, ``Q() objects`` make it possible to define and reuse conditions.
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This permits the :ref:`construction of complex database queries
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<complex-lookups-with-q>` using ``|`` (``OR``) and ``&`` (``AND``) operators;
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in particular, it is not otherwise possible to use ``OR`` in ``QuerySets``.
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``Prefetch()`` objects
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======================
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.. versionadded:: 1.7
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.. class:: Prefetch(lookup, queryset=None, to_attr=None)
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The ``Prefetch()`` object can be used to control the operation of
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:meth:`~django.db.models.query.QuerySet.prefetch_related()`.
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The ``lookup`` argument describes the relations to follow and works the same
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as the string based lookups passed to
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:meth:`~django.db.models.query.QuerySet.prefetch_related()`.
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The ``queryset`` argument supplies a base ``QuerySet`` for the given lookup.
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This is useful to further filter down the prefetch operation, or to call
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:meth:`~django.db.models.query.QuerySet.select_related()` from the prefetched
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relation, hence reducing the number of queries even further.
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The ``to_attr`` argument sets the result of the prefetch operation to a custom
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attribute.
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.. note::
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When using ``to_attr`` the prefetched result is stored in a list.
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This can provide a significant speed improvement over traditional
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``prefetch_related`` calls which store the cached result within a
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``QuerySet`` instance.
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