Example
Problem
Django querysets are evaluated in a lazy fashion. For example:
# models.py:
class Author(models.Model):
name = models.CharField(max_length=100)
class Book(models.Model):
author = models.ForeignKey(Author, related_name='books')
title = models.CharField(max_length=100)
# views.py
def myview(request):
# Query the database
books = Book.objects.all()
for book in books:
# Query the database on each iteration to get author (len(books) times)
# if there is 100 books, there will have 100 queries plus the initial query
book.author
# ...
# total : 101 queries
The code above causes django to query the database for the author of each book. This is inefficient, and it is better to only have a single query.
Solution
Use select_related on ForeignKey if you know that you will need to later access a ForeignKey field.
# views.py
def myview(request):
# Query the database.
books = Books.objects.select_related('author').all()
for book in books:
# Does not query the database again, since `author` is pre-populated
book.author
# ...
# total : 1 query
select_related can also be used on lookup fields:
# models.py:
class AuthorProfile(models.Model):
city = models.CharField(max_length=100)
class Author(models.Model):
name = models.CharField(max_length=100)
profile = models.OneToOneField(AuthorProfile)
class Book(models.Model):
author = models.ForeignKey(Author, related_name='books')
title = models.CharField(max_length=100)
# views.py
def myview(request):
books = Book.objects.select_related('author')\
.select_related('author__profile').all()
for book in books:
# Does not query database
book.author.name
# or
book.author.profile.city
# ...
# total : 1 query
