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