Ruby on rails 是否通过从组_中的每个组检索单个记录？

我正在努力让每个月在就业方面得分最高的企业的员工

架构如下所示：

class Business < ActiveRecord::Base
 has_many :employees
 has_many :placements, through: :employees

class Employee < ActiveRecord::Base
 attr_accessible :name
 belongs_to :business
 has_many :placements

class Placement < ActiveRecord::Base
 attr_accessible :month, :employee_id, :points
 belongs_to :employee

但是我得到了一个PostgreSQL

groupby

错误：

: SELECT "placements".* FROM "placements" INNER JOIN "employees" ON "placements"."employee_id" = "employees"."id" WHERE "employees"."business_id" = 43 GROUP BY month ORDER BY points DESC
ActiveRecord::StatementInvalid: PG::Error: ERROR:  column "placements.id" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: SELECT "placements".* FROM "placements" INNER JOIN "employee...

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任何帮助都将不胜感激。

当您对所选的任何列进行分组时，您需要将其包含在“分组人”中，或者在这些列上具有某种聚合功能。默认情况下，AR会缩回所有列，因此您可能希望使用

限制这些列。选择
当您收到错误时，始终显示错误的确切文本。在这种情况下，我认为您需要的可能是
HAVING
子句。谢谢Craig。我添加了错误。如果您没有在语句中添加“includes”，错误是否仍然存在？是，如果我删除。includes（：employee），我仍然会收到错误
: SELECT "placements".* FROM "placements" INNER JOIN "employees" ON "placements"."employee_id" = "employees"."id" WHERE "employees"."business_id" = 43 GROUP BY month ORDER BY points DESC
ActiveRecord::StatementInvalid: PG::Error: ERROR:  column "placements.id" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: SELECT "placements".* FROM "placements" INNER JOIN "employee...