django查询如何工作？

Question

我的模型设计如此

class Warehouse:
    name = ...
    sublocation = FK(Sublocation)

class Sublocation:
    name = ...
    city = FK(City)

class City:
    name = ..
    state = Fk(State)

现在如果我抛出一个查询.

wh = Warehouse.objects.value_list(['name', 'sublocation__name', 
    'sublocation__city__name']).first()

它会返回正确的结果,但内部会抛出多少查询？是django在一个请求中获取数据？

Answer 1

Django只对数据库进行一次查询,以获取您描述的数据.当你这样做时:

wh = Warehouse.objects.values_list(
    'name', 'sublocation__name', 'sublocation__city__name').first()

它转换为此查询:

SELECT "myapp_warehouse"."name", "myapp_sublocation"."name", "myapp_city"."name"
FROM "myapp_warehouse" INNER JOIN "myapp_sublocation"
ON ("myapp_warehouse"."sublocation_id" = "myapp_sublocation"."id")
INNER JOIN "myapp_city" ON ("myapp_sublocation"."city_id" = "myapp_city"."id")'

它在单个查询中获得结果.您可以在shell中计算查询数量,如下所示:

from django.db import connection as c, reset_queries as rq

In [42]: rq()

In [43]: len(c.queries)
Out[43]: 0

In [44]: wh = Warehouse.objects.values_list('name', 'sublocation__name', 'sublocation__city__name').first()

In [45]: len(c.queries)
Out[45]: 1