Django select_related不起作用

Question

我的Django select_related很奇怪

Models:
class Publisher(models.Model):
 name = models.CharField(max_length=100)
 class Meta:
      app_label = 'models'
      db_table = 'Publisher'
class Book(models.Model):
 name = models.CharField(max_length=100)
 publisher = models.OneToOneField(Publisher)
 class Meta:
      app_label = 'models'
     db_table = 'Book'

输出：

books = Book.objects.select_related('publisher').all()
print books.query
SELECT "Book"."id", "Book"."name", "Book"."publisher_id", "Publisher"."id", "Publisher"."name" FROM "Book" INNER JOIN "Publisher" ON ( "Book"."publisher_id" = "Publisher"."id" )
print books.values()
[{'publisher_id': 1, u'id': 1, 'name': u'rest framework'}]

Django会生成正确的查询，并在我执行它时检索数据。但是值不包含发布者

Answer 1

您有点误解了selected_related工作原理。参考django文档select_related：

select_related返回一个QuerySet，它将“遵循”外键关系，并在执行查询时选择其他相关对象数据。 这可以提高性能，从而导致单个更复杂的查询，但意味着以后使用外键关系将不需要数据库查询。

正如您已经确定的那样，添加select_related会使django选择相关对象的数据（在本例中为Publisher.id＆Publisher.name）。但是，该all()方法仍将仅返回BookQuerySet。

哪里是当您访问，这是非常有用Book的Publisher，Django将不需要再次查询数据库为Publisher：

# Hits the database.
# SELECT "Book"."id", "Book"."name", "Book"."publisher_id" ...
b = Book.objects.get(name='Twilight')

# Hits the database again to get the related Book object.
# SELECT "Publisher"."id", "Publisher"."name" ...
p = b.publisher

那是两个数据库查询，而select_related查找只是一个：

# Hits the database, but already includes Publisher data in the query
# SELECT "Book"."id", "Book"."name", "Book"."publisher_id", "Publisher"."id", "Publisher"."name" ...
b = Entry.objects.select_related('publisher').get(name='Twilight')

# Doesn't hit the database, because b.publisher has been prepopulated
# in the previous query.
p = b.publisher

（示例与django docs示例略有不同）