实战Django之Model操作之select_related 和 prefetch_related最佳实践（三）
这是本系列的最后一篇，主要是select_related() 和 prefetch_related() 的最佳实践。
主篇这里
第一篇在这里 讲例子和select_related()
第二篇在这里 讲prefetch_related()
4. 一些实例
选择哪个函数
如果我们想要获得所有家乡是湖北的人，最无脑的做法是先获得 湖北省，再获得湖北的所有城市，最后获得故乡是这个城市的人。就像这样：
-
>>> hb = Province.objects.get(name__iexact=
u"湖北省")
-
>>> people = []
-
>>>
for city
in hb.city_set.all():
-
... people.extend(city.birth.all())
-
...
prefetch_related() 或许是一个好的解决方法，让我们来看看。
-
>>> hb = Province.objects.prefetch_related(
"city_set__birth").objects.get(name__iexact=
u"湖北省")
-
>>> people = []
-
>>>
for city
in hb.city_set.all():
-
... people.extend(city.birth.all())
-
...
-
SELECT
`QSOptimize_province`.
`id`,
`QSOptimize_province`.
`name`
-
FROM
`QSOptimize_province`
-
WHERE
`QSOptimize_province`.
`name`
LIKE
'湖北省' ;
-
-
SELECT
`QSOptimize_city`.
`id`,
`QSOptimize_city`.
`name`,
`QSOptimize_city`.
`province_id`
-
FROM
`QSOptimize_city`
-
WHERE
`QSOptimize_city`.
`province_id`
IN (
1);
-
-
SELECT
`QSOptimize_person`.
`id`,
`QSOptimize_person`.
`firstname`,
`QSOptimize_person`.
`lastname`,
-
`QSOptimize_person`.
`hometown_id`,
`QSOptimize_person`.
`living_id`
-
FROM
`QSOptimize_person`
-
WHERE
`QSOptimize_person`.
`hometown_id`
IN (
1,
3);
嗯...看上去不错，但是3次查询么？倒过来查询可能会更简单？
>>> people = list(Person.objects.select_related("hometown__province").filter(hometown__province__name__iexact=u"湖北省"))
-
SELECT
`QSOptimize_person`.
`id`,
`QSOptimize_person`.
`firstname`,
`QSOptimize_person`.
`lastname`,
-
`QSOptimize_person`.
`hometown_id`,
`QSOptimize_person`.
`living_id`,
`QSOptimize_city`.
`id`,
-
`QSOptimize_city`.
`name`,
`QSOptimize_city`.
`province_id`,
`QSOptimize_province`.
`id`,
`QSOptimize_province`.
`name`
-
FROM
`QSOptimize_person`
-
INNER
JOIN
`QSOptimize_city`
ON (
`QSOptimize_person`.
`hometown_id` =
`QSOptimize_city`.
`id`)
-
INNER
JOIN
`QSOptimize_province`
ON (
`QSOptimize_city`.
`province_id` =
`QSOptimize_province`.
`id`)
-
WHERE
`QSOptimize_province`.
`name`
LIKE
'湖北省';
+----+-----------+----------+-------------+-----------+----+--------+-------------+----+--------+
| id | firstname | lastname | hometown_id | living_id | id | name | province_id | id | name | +----+-----------+----------+-------------+-----------+----+--------+-------------+----+--------+ | 1 | 张 | 三 | 3 | 1 | 3 | 十堰市 | 1 | 1 | 湖北省 |
| 2 | 李 | 四 | 1 | 3 | 1 | 武汉市 | 1 | 1 | 湖北省 | | 3 | 王 | 麻子 | 3 | 2 | 3 | 十堰市 | 1 | 1 | 湖北省 |
+----+-----------+----------+-------------+-----------+----+--------+-------------+----+--------+
3 rows in set (0.00 sec)
select_related()的效率要高于prefetch_related()。因此，最好在能用select_related()的地方尽量使用它，也就是说，对于ForeignKey字段，避免使用prefetch_related()。
联用
对于同一个QuerySet，你可以同时使用这两个函数。
在我们一直使用的例子上加一个model：Order （订单）
-
class Order(models.Model):
-
customer = models.ForeignKey(Person)
-
orderinfo = models.CharField(max_length=
50)
-
time = models.DateTimeField(auto_now_add =
True)
-
def __unicode__(self):
-
return self.orderinfo
-
>>> plist = Order.objects.prefetch_related(
'customer__visitation__province').get(id=
1)
-
>>>
for city
in plist.customer.visitation.all():
-
...
print city.province.name
-
...
-
SELECT
`QSOptimize_order`.
`id`,
`QSOptimize_order`.
`customer_id`,
`QSOptimize_order`.
`orderinfo`,
`QSOptimize_order`.
`time`
-
FROM
`QSOptimize_order`
-
WHERE
`QSOptimize_order`.
`id` =
1 ;
-
-
SELECT
`QSOptimize_person`.
`id`,
`QSOptimize_person`.
`firstname`,
`QSOptimize_person`.
`lastname`,
`QSOptimize_person`.
`hometown_id`,
`QSOptimize_person`.
`living_id`
-
FROM
`QSOptimize_person`
-
WHERE
`QSOptimize_person`.
`id`
IN (
1);
-
-
SELECT (
`QSOptimize_person_visitation`.
`person_id`)
AS
`_prefetch_related_val`,
`QSOptimize_city`.
`id`,
-
`QSOptimize_city`.
`name`,
`QSOptimize_city`.
`province_id`
-
FROM
`QSOptimize_city`
-
INNER
JOIN
`QSOptimize_person_visitation`
ON (
`QSOptimize_city`.
`id` =
`QSOptimize_person_visitation`.
`city_id`)
-
WHERE
`QSOptimize_person_visitation`.
`person_id`
IN (
1);
-
-
SELECT
`QSOptimize_province`.
`id`,
`QSOptimize_province`.
`name`
-
FROM
`QSOptimize_province`
-
WHERE
`QSOptimize_province`.
`id`
IN (
1,
2);
+----+-------------+---------------+---------------------+
| id | customer_id | orderinfo | time | +----+-------------+---------------+---------------------+ | 1 | 1 | Info of Order | 2014-08-10 17:05:48 |
+----+-------------+---------------+---------------------+
1 row in set (0.00 sec)
±—±----------±---------±------------±----------+
| id | firstname | lastname | hometown_id | living_id |
±—±----------±---------±------------±----------+
| 1 | 张 | 三 | 3 | 1 |
±—±----------±---------±------------±----------+
1 row in set (0.00 sec)
±----------------------±—±-------±------------+
| _prefetch_related_val | id | name | province_id |
±----------------------±—±-------±------------+
| 1 | 1 | 武汉市 | 1 |
| 1 | 2 | 广州市 | 2 |
| 1 | 3 | 十堰市 | 1 |
±----------------------±—±-------±------------+
3 rows in set (0.00 sec)
±—±-------+
| id | name |
±—±-------+
| 1 | 湖北省 |
| 2 | 广东省 |
±—±-------+
2 rows in set (0.00 sec)
更好的办法是先调用一次select_related()再调用prefetch_related()，最后再select_related()后面的表
-
>>> plist = Order.objects.select_related(
'customer').prefetch_related(
'customer__visitation__province').get(id=
1)
-
>>>
for city
in plist.customer.visitation.all():
-
...
print city.province.name
-
...
-
SELECT
`QSOptimize_order`.
`id`,
`QSOptimize_order`.
`customer_id`,
`QSOptimize_order`.
`orderinfo`,
-
`QSOptimize_order`.
`time`,
`QSOptimize_person`.
`id`,
`QSOptimize_person`.
`firstname`,
-
`QSOptimize_person`.
`lastname`,
`QSOptimize_person`.
`hometown_id`,
`QSOptimize_person`.
`living_id`
-
FROM
`QSOptimize_order`
-
INNER
JOIN
`QSOptimize_person`
ON (
`QSOptimize_order`.
`customer_id` =
`QSOptimize_person`.
`id`)
-
WHERE
`QSOptimize_order`.
`id` =
1 ;
-
-
SELECT (
`QSOptimize_person_visitation`.
`person_id`)
AS
`_prefetch_related_val`,
`QSOptimize_city`.
`id`,
-
`QSOptimize_city`.
`name`,
`QSOptimize_city`.
`province_id`
-
FROM
`QSOptimize_city`
-
INNER
JOIN
`QSOptimize_person_visitation`
ON (
`QSOptimize_city`.
`id` =
`QSOptimize_person_visitation`.
`city_id`)
-
WHERE
`QSOptimize_person_visitation`.
`person_id`
IN (
1);
-
-
SELECT
`QSOptimize_province`.
`id`,
`QSOptimize_province`.
`name`
-
FROM
`QSOptimize_province`
-
WHERE
`QSOptimize_province`.
`id`
IN (
1,
2);
+----+-------------+---------------+---------------------+----+-----------+----------+-------------+-----------+
| id | customer_id | orderinfo | time | id | firstname | lastname | hometown_id | living_id |
+----+-------------+---------------+---------------------+----+-----------+----------+-------------+-----------+
| 1 | 1 | Info of Order | 2014-08-10 17:05:48 | 1 | 张 | 三 | 3 | 1 |
+----+-------------+---------------+---------------------+----+-----------+----------+-------------+-----------+
1 row in set (0.00 sec)
±----------------------±—±-------±------------+
| _prefetch_related_val | id | name | province_id |
±----------------------±—±-------±------------+
| 1 | 1 | 武汉市 | 1 |
| 1 | 2 | 广州市 | 2 |
| 1 | 3 | 十堰市 | 1 |
±----------------------±—±-------±------------+
3 rows in set (0.00 sec)
±—±-------+
| id | name |
±—±-------+
| 1 | 湖北省 |
| 2 | 广东省 |
±—±-------+
2 rows in set (0.00 sec)
值得注意的是，可以在调用prefetch_related之前调用select_related，并且Django会按照你想的去做：先select_related，然后利用缓存到的数据prefetch_related。然而一旦prefetch_related已经调用，select_related将不起作用。
小结
- 因为select_related()总是在单次SQL查询中解决问题，而prefetch_related()会对每个相关表进行SQL查询，因此select_related()的效率通常比后者高。
- 鉴于第一条，尽可能的用select_related()解决问题。只有在select_related()不能解决问题的时候再去想prefetch_related()。
- 你可以在一个QuerySet中同时使用select_related()和prefetch_related()，从而减少SQL查询的次数。
- 只有prefetch_related()之前的select_related()是有效的，之后的将会被无视掉。
关于这两个函数，我能想到的东西目前只有这么多。不过基于一些个人原因，写第三篇时间比较短，写的有些仓促。如果什么时候又想起了什么，我会在这篇博文中添加。