
select_related and prefetch_related in Django
Jan. 9, 2023
In Django select_related
and prefetch_related
are Django ORM (Object-Relational Mapper) functions that allow you to retrieve related objects of a model in a single database query. This can improve the performance of your Django application by reducing the number of database queries that are needed to render a webpage.
select_related
is used to retrieve all the related objects of a model in a single database query. For example, if you have a model Person
that has a ForeignKey
field to a Country
model, you can use select_related
to retrieve all the Person
objects and their corresponding Country
objects in a single database query:
people = Person.objects.select_related('country').all()
prefetch_related
is similar to select_related
, but it is used to retrieve related objects of a model that has a ManyToManyField
or a OneToOneField
. It works by creating a separate database query for each ManyToManyField
or OneToOneField
of the model. For example, if you have a model Book
that has a ManyToManyField
to an Author
model, you can use prefetch_related
to retrieve all the Book
objects and their corresponding Author
objects in a single database query:
books = Book.objects.prefetch_related('authors').all()
Both select_related
and prefetch_related
can improve the performance of your Django application by reducing the number of database queries that are needed to render a webpage. However, they can also increase the memory usage of your application, as they retrieve all the related objects in a single query, so it is important to use them wisely.