减少 ManyToManyField 上的查询数量（n1 问题）

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

如果你知道稍后需要访问一个 ManyToManyField 字段的字段，请在 ManyToManyField 上使用 prefetch_related。

# 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 books

prefetch_related 也可用于查找字段：

# 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，如果不再次访问数据库，则无法更改所获取的数据。以下将执行额外查询，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 Django 1.7 中引入的 Prefetch 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