PG::InvalidColumnReference: ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list

Question

我有一长串关联、连接、排序依据等，最终 select 来自我的一个 rails 模型。在一天结束时，我需要结果是唯一的和排序的。我不关心 SELECT 语句中使用了哪些列，ORDER BY 中出现了什么，等等（这些都根据用户 selected 的过滤选项而改变），我只关心查询中的顶级 model/table 是唯一的（基于 id）。

对于背景，widgets 是主要的 table，我们正在加入 widget_steps，这是在 Rails 3（公司正在尝试尽快升级但这就是他们目前所坚持的）

这是正在生成的查询和错误：

PG::InvalidColumnReference: ERROR:  for SELECT DISTINCT, ORDER BY expressions must appear in select list
LINE 1: ...completed_at" IS NULL)) ORDER BY sequential DESC, widget_s...
                                                             ^
: SELECT  DISTINCT "widgets".* FROM "widgets" INNER JOIN "widget_steps" ON "widget_steps"."widget_id" = "widgets"."id" INNER JOIN "widget_steps" "active_steps_widgets" ON "active_steps_widgets"."id" = "widgets"."active_widget_step_id" WHERE "widgets"."account_id" = 1 AND "widgets"."completed_at" IS NULL AND (("widgets"."sequential" = 't' AND "widget_steps"."assigned_to" = 5 AND "widget_steps"."id" = "widgets"."active_widget_step_id" AND "widget_steps"."completed_at" IS NULL) OR ("widgets"."sequential" = 'f' AND "widget_steps"."assigned_to" = 5 AND "widget_steps"."completed_at" IS NULL)) ORDER BY sequential DESC, widget_steps.name ASC LIMIT 10 OFFSET 0
Completed 500 Internal Server Error in 52.3ms

ActiveRecord::StatementInvalid - PG::InvalidColumnReference: ERROR:  for SELECT DISTINCT, ORDER BY expressions must appear in select list
LINE 1: ...completed_at" IS NULL)) ORDER BY sequential DESC, widget_s...
                                                             ^
: SELECT  DISTINCT "widgets".* FROM "widgets" INNER JOIN "widget_steps" ON "widget_steps"."widget_id" = "widgets"."id" INNER JOIN "widget_steps" "active_steps_widgets" ON "active_steps_widgets"."id" = "widgets"."active_widget_step_id" WHERE "widgets"."account_id" = 1 AND "widgets"."completed_at" IS NULL AND (("widgets"."sequential" = 't' AND "widget_steps"."assigned_to" = 5 AND "widget_steps"."id" = "widgets"."active_widget_step_id" AND "widget_steps"."completed_at" IS NULL) OR ("widgets"."sequential" = 'f' AND "widget_steps"."assigned_to" = 5 AND "widget_steps"."completed_at" IS NULL)) ORDER BY sequential DESC, widget_steps.name ASC LIMIT 10 OFFSET 0:

为什么会这样？ postgres 认为什么如此模棱两可？为什么像这样的查询在 MySQL 中总是工作正常，但让 postgres 窒息。

我试过：

在链的末尾指定 .select([everything mentioned in the order by]).uniq
在链的末尾指定 .uniq 而不进行自定义 select
编写一些自定义 AREL 以尝试将所有这些嵌入到子查询中，然后在此之外执行 .uniq 或 .order（无法正常工作）
在 postgres 之外执行 .uniq（这会因为分页而中断......由于重复项被删除，您最终可能会得到一些只有 1 或 2 个项目的页面）
哭泣

Answer 1

您需要将 widget_steps.name 添加到所选列的列表中：

SELECT  DISTINCT "widgets".*, "widget_steps.name" FROM "widgets" INNER JOIN "widget_steps" ON "widget_steps"."widget_id" = "widgets"."id" INNER JOIN "widget_steps" "active_steps_widgets" ON "active_steps_widgets"."id" = "widgets"."active_widget_step_id" WHERE "widgets"."account_id" = 1 AND "widgets"."completed_at" IS NULL AND (("widgets"."sequential" = 't' AND "widget_steps"."assigned_to" = 5 AND "widget_steps"."id" = "widgets"."active_widget_step_id" AND "widget_steps"."completed_at" IS NULL) OR ("widgets"."sequential" = 'f' AND "widget_steps"."assigned_to" = 5 AND "widget_steps"."completed_at" IS NULL)) ORDER BY sequential DESC, widget_steps.name ASC LIMIT 10 OFFSET 0

这不会改变您的查询逻辑，并且可以正常工作。

在 Rails 中，您可以使用 select 方法来设置所选列的列表：

Widget.select('"widgets".*, "widget_steps.name"')

希望这会有所帮助。

Answer 2

建议：

您可以在 Widget class 中添加 has_many :steps，并使用 Widget.includes(:steps) 执行一个查询，其中包含按 widget_steps 列排序的 Order 子句

Answer 3

另一个可能适用于某些情况（例如 has_and_belongs_to_many 类型关系）的错误选项是取消范围顺序：

class Resource < ApplicationRecord
  has_and_belongs_to_many :things
  scope :with_blue_things, -> { joins(:things).where(:things => {:color => :blue}).unscope(:order).distinct }

这应该让你做所有这些

Resource.with_blue_things
Resource.with_blue_things.count
Resource.with_blue_things.order(:name).count

PG::InvalidColumnReference: ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list

PG::InvalidColumnReference: ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list

ruby-on-rails

postgresql

activerecord

arel