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

Question

我有很长的关联链、连接链、排序依据等，它们最终是从我的一个 Rails 模型中进行选择的。在一天结束时，我需要结果是唯一的和排序的。我不在乎在 SELECT 语句中使用了哪些列，在 ORDER BY 中出现了什么（这些都根据用户选择的过滤选项而变化），我只关心顶层模型/表查询是唯一的（基于 id）。

对于背景，widgets是主表，我们正在加入widget_steps，这是在 Rails 3 中（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:

为什么这是一回事？postgres 认为什么对此如此模棱两可？为什么像这样的查询在 MySQL 中总是可以正常工作，但会使 postgres 窒息。

我试过了：

1. .select([everything mentioned in the order by]).uniq在链的末尾指定
2. .uniq在链的末尾指定而不进行自定义选择
3. 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（无法使其正常工作）
4. 在 postgres 之外执行 .uniq （这会因为分页而中断......你最终可能会得到一些页面，因为重复项被删除，其中只有 1 或 2 个项目）
5. 哭

Answer 1

您需要添加widget_steps.name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

这不应该改变您查询的逻辑并且可以正常工作。

在 Rails 中，您可以使用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。