Question

我想检索具有许多活动的联系人，其中后者的最大completed_date是6个月前。让我说清楚：

user = User.first
user.contacts.first.activities.maximum(:completed_date)
# SELECT MAX("activities"."completed_date") AS max_id FROM "activities" WHERE "activities"."user_id" = $1  [["user_id", 12]]
=> 2014-03-18 09:06:54 UTC

多数民众赞成。现在我想为WHERE查询使用该子句，但似乎我不能：

user.contacts.joins(:activities)
  .where('MAX("activities"."completed_date") < ?', Time.now - 6.months)
# SELECT "contacts".* FROM "contacts"
# INNER JOIN "activities" ON "activities"."contact_id" = "contacts"."id"
# WHERE "contacts"."user_id" = $1 AND (MAX("activities"."completed_date") <= '2013-09-23 05:55:21.191254')  [["user_id", 12]]

#=> PG::GroupingError: ERROR:  aggregate functions are not allowed in WHERE
#   LINE 1: ...ntacts"."id" WHERE "contacts"."user_id" = $1 AND (MAX("activ...

我该怎么做？

Answer 1

由于MAX聚合函数调用了WHERE子句，因此抱怨。

要避免此问题，请MAX SELECT与AS混淆WHERE。然后使用user.contacts.select('*, MAX("activities"."completed_date") AS max_complete_date').joins(:activities) .where('max_complete_date < ?', Time.now - 6.months)中的别名。

HAVING

修改

我道歉，您应该使用user.contacts.joins(:activities) .having('MAX("activities"."completed_date") < ?', Time.now - 6.months).group("contacts.id")代替。

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ActiveRecord / Postgres：WHERE中不允许聚合函数

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