Django 减少对ForeignKey字段的查询数量（n + 1个问题）
示例
问题
Django查询集以惰性方式求值。例如：
# 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上面的代码使django在数据库中查询每本书的作者。这样效率低下，最好只包含一个查询。
解
使用select_related上ForeignKey，如果你知道你将需要以后访问一个ForeignKey领域。
# 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 queryselect_related也可以用于查找字段：
# 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
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