PG :: InvalidColumnReference：错误：对于SELECT DISTINCT，ORDER BY表达式必须出现在选择列表中

Question

我有一个很长的关联链，连接，排序等，最终从我的一个rails模型中进行选择。在一天结束时，我需要结果是独特的和排序的。我不在乎SELECT语句中使用了哪些列，ORDER BY中出现了什么（这些都是基于用户选择的过滤选项而改变的），我只关心顶层模型/表格查询是唯一的（基于id）。

有关背景，widgets是主表，我们正在与widget_steps的加盟，这是Rails 3的（公司正在努力尽快升级，但是这就是他们坚持的时刻是什么）

这里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: 

这是为什么？ postgres认为对此如此含糊不清？为什么像这样的查询在MySQL中总能正常工作，但会导致postgres窒息。

我曾尝试：

1. 在链
2. 指定.uniq在链的末端没有做一个自定义的结束指定.select([everything mentioned in the order by]).uniq选择
3. 编写一些自定义的AREL尝试嵌入所有这在子查询中，然后执行.uniq或.order以外的这个（不能得到这个工作）
4. 做postgres之外的.uniq（这是因为分页而中断...你可以最终一些页面只有1个或者被删除2项，因为重复的对他们）
5. 哭

Answer 1

您需要添加widget_steps.name到所选列的列表：

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 

这不应该改变您的查询的逻辑，将工作精细。

在Rails中你可以使用select方法来设置选定列的列表：

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

希望这会有所帮助。