Django 减少对ManyToManyField的查询数量（n + 1个问题）
示例
问题
# models.py:
class Library(models.Model):
name = models.CharField(max_length=100)
books = models.ManyToManyField(Book)
class Book(models.Model):
title = models.CharField(max_length=100)# views.py
def myview(request):
# Query the database.
libraries = Library.objects.all()
# Query the database on each iteration (len(author) times)
# if there is 100 librairies, there will have 100 queries plus the initial query
for library in libraries:
books = library.books.all()
books[0].title
# ...
# total : 101 queries解
使用prefetch_related上ManyToManyField，如果你知道你需要访问后场是一个ManyToManyField场。
# views.py
def myview(request):
# Query the database.
libraries = Library.objects.prefetch_related('books').all()
# Does not query the database again, since `books` is pre-populated
for library in libraries:
books = library.books.all()
books[0].title
# ...
# total : 2 queries - 1 for libraries, 1 for booksprefetch_related也可以用于查找字段：
# models.py:
class User(models.Model):
name = models.CharField(max_length=100)
class Library(models.Model):
name = models.CharField(max_length=100)
books = models.ManyToManyField(Book)
class Book(models.Model):
title = models.CharField(max_length=100)
readers = models.ManyToManyField(User) # views.py
def myview(request):
# Query the database.
libraries = Library.objects.prefetch_related('books', 'books__readers').all()
# Does not query the database again, since `books` and `readers` is pre-populated
for library in libraries:
for book in library.books.all():
for user in book.readers.all():
user.name
# ...
# total : 3 queries - 1 for libraries, 1 for books, 1 for readers但是，一旦执行了查询集，就无法更改获取的数据，而无需再次击中数据库。下面将执行额外的查询，例如：
# views.py
def myview(request):
# Query the database.
libraries = Library.objects.prefetch_related('books').all()
for library in libraries:
for book in library.books.filter(title__contains="Django"):
print(book.name)可以使用PrefetchDjango1.7中引入的对象优化以下内容：
from django.db.models import Prefetch
# views.py
def myview(request):
# Query the database.
libraries = Library.objects.prefetch_related(
Prefetch('books', queryset=Book.objects.filter(title__contains="Django")
).all()
for library in libraries:
for book in library.books.all():
print(book.name) # Will print only books containing Django for each library