减少 ForeignKey 字段上的查询次数（n1 个问题）

问题

Django 查询集以惰性方式进行评估。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。这是低效的，最好只有一个查询。

解

如果你知道以后需要访问 ForeignKey 字段，请在 ForeignKey 上使用 select_related。

# 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 也可用于查找字段：

# 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