我在用户和捐赠之间有一个简单的关系,即用户有很多捐赠,捐赠属于用户.我想做的是获取最近捐赠的用户列表. @H_403_2@这是我正在尝试的：

@H_403_2@首先,我想获得uniq用户的总数,这是按预期工作的：

> User.joins(:donations).order('donations.created_at').uniq.count
   (3.2ms)  SELECT DISTINCT COUNT(DISTINCT "users"."id") FROM "users" INNER JOIN "donations" ON "donations"."user_id" = "users"."id"
 => 384

@H_403_2@接下来,当我删除count方法时,出现“ORDER BY表达式必须出现在选择列表中”的错误：

> User.joins(:donations).order('donations.created_at').uniq
  User Load (0.9ms)  SELECT DISTINCT "users".* FROM "users" INNER JOIN "donations" ON "donations"."user_id" = "users"."id" ORDER BY donations.created_at
PG::InvalidColumnReference: ERROR:  for SELECT DISTINCT,ORDER BY expressions must appear in select list
LINE 1: ...ON "donations"."user_id" = "users"."id"  ORDER BY donations....

@H_403_2@然后我尝试通过显式设置SELECT子句来修复Postgres错误,该子句乍一看似乎有效：

> User.select('DISTINCT "users".id,"users".*,"donations".created_at').joins(:donations).order('donations.created_at')
  User Load (17.6ms)  SELECT DISTINCT "users".id,"donations".created_at FROM "users" INNER JOIN "donations" ON "donations"."user_id" = "users"."id" ORDER BY donations.created_at

@H_403_2@但是,返回的记录数不考虑DISTINCT语句并返回692条记录：

> _.size
 => 692

@H_403_2@如何根据捐赠的created_at时间戳排序,获得预期的结果数(384)？

User.select('users.*,MAX(donations.created_at) as most_recent_donation').
  joins(:donations).order('most_recent_donation desc').group('users.id')