PostgreSQL Rails has_many :through / collection_singular_ids / :order 问题

Question

在迁移到heroku的过程中，我只有在使用PostgreSQL时才会出现奇怪的错误（在Mysql中可以正常工作）

当我执行@user.county_ids 时出现以下错误：

ActiveRecord::StatementInvalid: PGError: ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list

LINE 1: ...id" WHERE ("activity_areas".user_id = 1) ORDER BY counties.n...

生成的sql请求是：

SELECT DISTINCT "activity_areas".county_id FROM "activity_areas" INNER JOIN "counties" ON "counties"."id" = "activity_areas"."county_id" WHERE ("activity_areas".user_id = 1) ORDER BY counties.name ASC

最后是模型：

class User < ActiveRecord::Base
  has_many :activity_areas
  has_many :counties, :through => :activity_areas
end

class ActivityArea < ActiveRecord::Base
  belongs_to :user
  belongs_to :county

  default_scope joins(:county).order("counties.name ASC")
end

class County < ActiveRecord::Base
  has_many :activity_areas
  has_many :users, :through => :activity_areas

  default_scope :order => 'name ASC'
end

关于如何解决这个问题的任何想法？ 谢谢，

Answer 1

对于 PostgreSQL，请确保 order by 子句中的元素也存在于 select 子句中。 MySQL 对这条规则有点宽容:)

尝试将活动区域模型中的默认范围更改为

default_scope select('counties.name').joins(:county).order("counties.name ASC")

这应该会生成一个类似的 SQL

SELECT DISTINCT "activity_areas".county_id, counties.name FROM "activity_areas"...