PG :: InvalidColumnReference：錯誤：對於SELECT DISTINCT，ORDER BY表達式必須出現在選擇列表中

Question

我有一個很長的關聯鏈，連接，排序等，最終從我的一個rails模型中進行選擇。在一天結束時，我需要結果是獨特的和排序的。我不在乎SELECT語句中使用了哪些列，ORDER BY中出現了什麼（這些都是基於用戶選擇的過濾選項而改變的），我只關心頂層模型/表格查詢是唯一的（基於id）。

有關背景，widgets是主表，我們正在與widget_steps的加盟，這是Rails 3的（公司正在努力盡快升級，但是這就是他們堅持的時刻是什麼）

這裏是正在生成的查詢和錯誤：

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: 

這是爲什麼？ postgres認爲對此如此含糊不清？爲什麼像這樣的查詢在MySQL中總能正常工作，但會導致postgres窒息。

我曾嘗試：

1. 在鏈
2. 指定.uniq在鏈的末端沒有做一個自定義的結束指定.select([everything mentioned in the order by]).uniq選擇
3. 編寫一些自定義的AREL嘗試嵌入所有這在子查詢中，然後執行.uniq或.order以外的這個（不能得到這個工作）
4. 做postgres之外的.uniq（這是因爲分頁而中斷...你可以最終一些頁面只有1個或者被刪除2項，因爲重複的對他們）
5. 哭

Answer 1

您需要添加widget_steps.name到所選列的列表：

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 

這不應該改變您的查詢的邏輯，將工作精細。

在Rails中你可以使用select方法來設置選定列的列表：

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

希望這會有所幫助。