Question

在SQL中，条件的放置（无论是在ON子句中还是WHERE子句中）都会影响结果。 Django中的filter()似乎将条件放在WHERE子句中。

说我有这些模型：

class Nations(models.Model):
    deleted = models.BooleanField(default=False)


class People(models.Model):
    nation = models.ForeignKey(Nations, on_delete=models.CASCADE)
    deleted = models.BooleanField(default=False)

我想找到没有人的国家，我可以这样做：

SELECT nations.id, nations.deleted
FROM nations
LEFT JOIN people
   ON people.nation_id = nations.id
WHERE people.id IS NULL;

但是，可以说人们可以被软删除，我想找到其中没有不败之民的国家。我会的：

SELECT nations.id, nations.deleted
FROM nations
LEFT JOIN people
   ON people.nation_id = nations.id
   AND people.deleted IS FALSE
WHERE people.id IS NULL;

但是，在Django中，如果我这样做：

Nations.objects.filter(people__id__isnull=True, people__deleted=False)

这等同于查询：

SELECT nations.id, nations.deleted
FROM nations
LEFT JOIN people
   ON people.nation_id = nations.id
WHERE people.id IS NULL
   AND people.deleted IS FALSE;

与所需查询不同，该查询将不包括仅删除人员的国家！如何让Django将软删除检查移到ON子句中？

Answer 1

自Django 2.0以来，有一个鲜为人知的功能，名为FilteredRelation，该功能允许向关系中添加其他过滤器。结果是联接的ON子句中的附加条件：

nations = (Nations.objects
    .annotate(no_deleted=FilteredRelation(
        'people',
        condition=Q(people__deleted=False)))
    .filter(no_deleted=None)
)

print(str(nations.query))

生成的SQL看起来很像您所追求的：

SELECT "app_nations"."id",
       "app_nations"....
FROM "app_nations"
LEFT OUTER JOIN "app_people" no_deleted ON 
    ("app_nations"."id" = no_deleted."nations_id"
     AND (no_deleted."deleted" = FALSE))
WHERE no_deleted."id" IS NULL

Answer 2

在阅读您的最后一句话时，您似乎想要包括没有人的国家和只有被删除的人的国家。要做到这一点，您只需exclude那些没有删除人员的国家即可：

qs = Nations.objects.exclude(people__deleted=False)

如何将Django查询过滤器放在ON子句而不是WHERE子句中？

2 个答案: