Django Count和Sum批注相互干扰

这不是Django ORM的问题，这只是关系数据库的工作方式。当您构建简单的查询集时，例如

Player.objects.annotate(weapon_count=Count('unit_set__weapon_set'))

要么

Player.objects.annotate(rarity_sum=Sum('unit_set__rarity'))

ORM不正是你希望它做什么-加入

Player

Weapon

SELECt "sandbox_player"."id", "sandbox_player"."name", COUNT("sandbox_weapon"."id") AS "weapon_count"FROM "sandbox_player"LEFT OUTER JOIN "sandbox_unit"     ON ("sandbox_player"."id" = "sandbox_unit"."player_id")LEFT OUTER JOIN "sandbox_weapon"     ON ("sandbox_unit"."id" = "sandbox_weapon"."unit_id")GROUP BY "sandbox_player"."id", "sandbox_player"."name"

或

Player

Unit

SELECt "sandbox_player"."id", "sandbox_player"."name", SUM("sandbox_unit"."rarity") AS "rarity_sum"FROM "sandbox_player"LEFT OUTER JOIN "sandbox_unit" ON ("sandbox_player"."id" = "sandbox_unit"."player_id")GROUP BY "sandbox_player"."id", "sandbox_player"."name"

并对其执行

COUNT

SUM

请注意，尽管第一个查询在三个表之间具有两个联接，但是中间表

Unit

SELECt

GROUPBY

Unit

Player

Weapon

现在，如果您查看第三个查询集，事情将变得更加复杂。再次，如在第一个查询中一样，联接位于三个表之间，但现在由于存在以下汇总而

Unit

SELECT``SUM``Unit.rarity

SELECT "sandbox_player"."id",       "sandbox_player"."name",       COUNT(DISTINCT "sandbox_weapon"."id") AS "weapon_count",       SUM("sandbox_unit"."rarity")          AS "rarity_sum"FROM "sandbox_player"         LEFT OUTER JOIN "sandbox_unit" ON ("sandbox_player"."id" = "sandbox_unit"."player_id")         LEFT OUTER JOIN "sandbox_weapon" ON ("sandbox_unit"."id" = "sandbox_weapon"."unit_id")GROUP BY "sandbox_player"."id", "sandbox_player"."name"

这是第二和第三查询之间的关键区别。在第二个查询，要加入

Player

Unit

Unit

但在第三个查询要加入

Player

Unit

Unit

Weapon

Unit

Unit

让我们看一个简单的例子：

insert into sandbox_player values (1, "player_1");insert into sandbox_unit values(1, 10, 1);insert into sandbox_weapon values (1, 1), (2, 1);

一个玩家，一个单位和两个引用相同单位的武器。

确认问题存在：

>>> from sandbox.models import Player>>> from django.db.models import Count, Sum>>> Player.objects.annotate(weapon_count=Count('unit_set__weapon_set')).values()<QuerySet [{'id': 1, 'name': 'player_1', 'weapon_count': 2}]>>>> Player.objects.annotate(rarity_sum=Sum('unit_set__rarity')).values()<QuerySet [{'id': 1, 'name': 'player_1', 'rarity_sum': 10}]>>>> Player.objects.annotate(...     weapon_count=Count('unit_set__weapon_set', distinct=True),...     rarity_sum=Sum('unit_set__rarity')).values()<QuerySet [{'id': 1, 'name': 'player_1', 'weapon_count': 2, 'rarity_sum': 20}]>

从该示例可以很容易地看出问题是在组合查询中该单元将被列出两次，而引用该单元的每种武器都将被列出一次：

sqlite> SELECt "sandbox_player"."id",   ...>        "sandbox_player"."name",   ...>        "sandbox_weapon"."id",   ...>        "sandbox_unit"."rarity"   ...> FROM "sandbox_player"   ...>          LEFT OUTER JOIN "sandbox_unit" ON ("sandbox_player"."id" = "sandbox_unit"."player_id")   ...>          LEFT OUTER JOIN "sandbox_weapon" ON ("sandbox_unit"."id" = "sandbox_weapon"."unit_id");id          name        id          rarity    ----------  ----------  ----------  ----------1player_1    110        1player_1    210

正如@ivissani所提到的，最简单的解决方案之一是为每个聚合编写子查询：

>>> from django.db.models import Count, IntegerField, OuterRef, Subquery, Sum>>> weapon_count = Player.objects.annotate(weapon_count=Count('unit_set__weapon_set')).filter(pk=OuterRef('pk'))>>> rarity_sum = Player.objects.annotate(rarity_sum=Sum('unit_set__rarity')).filter(pk=OuterRef('pk'))>>> qs = Player.objects.annotate(...     weapon_count=Subquery(weapon_count.values('weapon_count'), output_field=IntegerField()),...     rarity_sum=Subquery(rarity_sum.values('rarity_sum'), output_field=IntegerField())... )>>> qs.values()<QuerySet [{'id': 1, 'name': 'player_1', 'weapon_count': 2, 'rarity_sum': 10}]>

产生以下SQL

SELECt "sandbox_player"."id", "sandbox_player"."name", (    SELECT COUNT(U2."id") AS "weapon_count"    FROM "sandbox_player" U0     LEFT OUTER JOIN "sandbox_unit" U1        ON (U0."id" = U1."player_id")    LEFT OUTER JOIN "sandbox_weapon" U2         ON (U1."id" = U2."unit_id")    WHERe U0."id" = ("sandbox_player"."id")     GROUP BY U0."id", U0."name") AS "weapon_count", (    SELECt SUM(U1."rarity") AS "rarity_sum"    FROM "sandbox_player" U0    LEFT OUTER JOIN "sandbox_unit" U1        ON (U0."id" = U1."player_id")    WHERe U0."id" = ("sandbox_player"."id")GROUP BY U0."id", U0."name") AS "rarity_sum"FROM "sandbox_player"