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Django条件子查询聚合

django-subquery django-annotate django

6

Eldamir Henri Siponen · 技术社区 · 6 年前

我的模型结构的一个简化示例是

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,我想显示一个显示公司的表,其中一列将包含特定类型的部门数,例如。 type=10 .目前,这是通过 Corporation 检索这些数据的模型,例如

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,这绝对是谋杀的表现,由于N+1的问题。

我试着用 select_related ,请 prefetch_related ,请 annotate ,和 subquery 但是我没能得到我需要的结果。

理想情况下,每个 公司 在查询集中,应使用整数注释 type_10_count 它反映了这类部门的数量。

我相信我可以用原始SQL做点什么 .extra() 但是文档宣布它将被否决(我在Django 1.11上)

编辑:原始SQL解决方案示例

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
""")

2 回复 | 直到 6 年前

1

5

Alexandr Tatarinov 6 年前

Subquery

# 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')
    )
)

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"

1. GROUP BY using .values and .annotate

2. order_by() problems

3. Subquery

2

3

solarissmoke 6 年前

你应该可以用 Case()

from django.db.models import Case, IntegerField, Sum, When, Value

Corporation.objects.annotate(
    type_10_count=Sum(
        Case(
            When(division__department__type=10, then=Value(1)),
            default=Value(0),
            output_field=IntegerField()
        )
    )
)