Django：外键数据库查询的数量

Question

我想澄清一些事情，以便我可以通过 psycopg2 适配器优化我对 Postgres 的 Django ORM 查询。

假设我有三个模型：

from django.db import models   

class City(models.Model):
     #
     pass

class Person(models.Model):
     hometown = models.ForeignKey(City)

class Book(models.Model):
     author = models.ForeignKey(Person)

现在让我们进行以下查询：

book = Book.objects.get(pk = 4)

现在如果我想知道作者的原始 ID，我需要做的就是这个

>>> raw_id = book.author_id
>>> print raw_id
3

我的问题是， book.author_id 是否再次访问数据库，或者它只是从内存中获取它，因为它已经从 get 查询中获取了？据我了解，这个查询应该只有一次行程正确吗？

Answer 1

你是对的，book.author_id 不会再次访问数据库，外键 ID 存储在 book 中。如果您确实使用 DEBUG=True 设置进入 shell，您可以自行查看：

>>> from django.db import connection

>>> connection.queries
[]

>>> book = Book.objects.get(pk=4)

>>> connection.queries
[{u'sql': u'SELECT "yourapp_book"."id", "yourapp_book"."author_id" FROM "yourapp_book" WHERE "yourapp_book"."id" = 4 ',
u'time': u'0.003'}]

>>> book.author_id
7

>>> connection.queries
[{u'sql': u'SELECT "yourapp_book"."id", "yourapp_book"."author_id" FROM "yourapp_book" WHERE "yourapp_book"."id" = 4 ',
u'time': u'0.003'}]

>>> book.author
<Author: Mark Twain>

>>> connection.queries
[{u'sql': u'SELECT "yourapp_book"."id", "yourapp_book"."author_id" FROM "yourapp_book" WHERE "yourapp_book"."id" = 4 ',
 u'time': u'0.003'},
 {u'sql': u'SELECT "yourapp_author"."id", "yourapp_author"."name" FROM "yourapp_author" WHERE "yourapp_author"."id" = 7 ',
 u'time': u'0.001'}]

connection.queries 列出了在DEBUG 开启时通过 django shell 中的 ORM 发送的所有 SQL 查询。请注意，即使在执行 book.author_id 之后也只进行了一次查询，但是当我们执行 book.author 时进行了第二次查询，因为 django 必须返回数据库以获取要显示的作者姓名。

您可以通过book = Book.objects.select_related('author').get(pk=4) 选择图书来避免第二次数据库查询。

>>> connection.queries
[]

>>> book = Book.objects.select_related('author').get(pk=4)

>>> connection.queries
[{u'sql': u'SELECT "yourapp_book"."id", "yourapp_book"."author_id", 
  "yourapp_author"."id", "yourapp_author"."name"
  FROM "yourapp_book" LEFT OUTER JOIN "yourapp_author" ON 
  ( "yourapp_book"."author_id" = "yourapp_author"."id" ) WHERE "yourapp_book"."id" = 4 ',
  u'time': u'0.003'}]

>>> book.author
<Author: Mark Twain>

>>> connection.queries
[{u'sql': u'SELECT "yourapp_book"."id", "yourapp_book"."author_id", 
  "yourapp_author"."id", "yourapp_author"."name"
  FROM "yourapp_book" LEFT OUTER JOIN "yourapp_author" ON 
  ( "yourapp_book"."author_id" = "yourapp_author"."id" ) WHERE "yourapp_book"."id" = 4 ',
  u'time': u'0.003'}]

我还应该注意，对于在测试期间调试正在运行的 django 应用程序中实际进行的查询数量，django-debug-toolbar 工具是无价的。