PostgreSQL Rails has_many :through / collection_singular_ids / :order 问题

Question

在迁移到heroku的过程中，只有当我使用PostgreSQL时才会出现奇怪的错误（在Mysql中工作正常）

当我执行@user.county_ids时，我收到以下错误：

ActiveRecord:: StatementInvalid: PGError: ERROR: 对于 SELECT DISTINCT, ORDER BY 表达式必须出现在选择列表中

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

生成的 sql 请求为：

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) 排序依据counties.name ASC

以及最后的模型：

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

知道如何解决这个问题吗？ 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对此规则有点宽松:)

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")

这应该生成类似的SQL

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