Django条件子查询聚合

Question

我的模型结构的简化示例是

class Corporation(models.Model):
    ...

class Division(models.Model):
    corporation = models.ForeignKey(Corporation)

class Department(models.Model):
    division = models.ForeignKey(Division)
    type = models.IntegerField()

现在我想显示一个表格,显示公司,其中列将包含某种类型的部门数量,例如type=10.目前,这是通过Corporation模型上的帮助程序实现的,例如,检索那些帮助程序

class Corporation(models.Model):
    ...
    def get_departments_type_10(self):
        return (
            Department.objects
            .filter(division__corporation=self, type=10)
            .count()
        )

这里的问题是,由于N + 1问题,这绝对是谋杀性能.

我试图用接近这个问题select_related,prefetch_related,annotate,和subquery,但我没有带能够得到我需要的结果.

理想情况下,查询Corporation集中的每个应该使用一个整数进行注释,该整数type_10_count反映了该类型的部门数量.

我确信我可以用原始sql做一些事情.extra(),但是文档宣布它将被弃用(我在Django 1.11上)

编辑:原始sql解决方案的示例

corps = Corporation.objects.raw("""
SELECT
*,
(
    SELECT COUNT(*)
    FROM foo_division div ON div.corporation_id = c.id
    JOIN foo_department dept ON dept.division_id = div.id
    WHERE dept.type = 10
) as type_10_count
FROM foo_corporation c
""")

Answer 1

我认为Subquery我们可以通过此代码获得类似于您提供的SQL

# Get amount of departments with GROUP BY division__corporation [1]
# .order_by() will remove any ordering so we won't get additional GROUP BY columns [2]
departments = Department.objects.filter(type=10).values(
    'division__corporation'
).annotate(count=Count('id')).order_by()

# Attach departments as Subquery to Corporation by Corporation.id.
# Departments are already grouped by division__corporation
# so .values('count') will always return single row with single column - count [3]
departments_subquery = departments.filter(division__corporation=OuterRef('id'))
corporations = Corporation.objects.annotate(
    departments_of_type_10=Subquery(
        departments_subquery.values('count'), output_field=IntegerField()
    )
)

生成的SQL是

SELECT "corporation"."id", ... (other fields) ...,
  (
    SELECT COUNT("division"."id") AS "count"
    FROM "department"
    INNER JOIN "division" ON ("department"."division_id" = "division"."id") 
    WHERE (
      "department"."type" = 10 AND
      "division"."corporation_id" = ("corporation"."id")
    ) GROUP BY "division"."corporation_id"
  ) AS "departments_of_type_10"
FROM "corporation"

这里的一些问题是子查询对于大表来说可能很慢.但是,数据库查询优化器可以足够智能地将子查询提升为OUTER JOIN,至少我听说PostgreSQL会这样做.

1. GROUP BY使用.values和.annotate

2. order_by()问题

3.子查询