Question

你好我发现了很多关于如何使用LEFT OUTER JOIN的例子，但我似乎无法找到一种方法来访问我加入的内容。这就是我的意思：

List.featured.
  joins(
    "LEFT OUTER JOIN follows ON (
      follows.followable_id = lists.id AND
      follows.followable_type = 'List' AND
      follows.user_id = #{current_user.id})"
  ).map { |list|
    list.follows # <-- returns all follows, not only the ones from current_user
    ...

在示例中，我通过连接获得了以下（似乎），但是我如何访问它们？ follows关系只会给我所有关注列表。

或许我的思绪模糊不清:)谢谢！

Answer 1

接下来要急切加载，您可以拨打includes()：

List.featured.includes(:follows).where(:follows => { :user_id => current_user.id })

它生成如下查询：

SELECT
  "lists"."id"          AS t0_r0,
  "lists"."is_featured" AS t0_r1,
  "lists"."created_at"  AS t0_r2,
  "lists"."updated_at"  AS t0_r3,
  "follows"."id"              AS t1_r0,
  "follows"."followable_id"   AS t1_r1,
  "follows"."followable_type" AS t1_r2,
  "follows"."user_id"         AS t1_r3,
  "follows"."created_at"      AS t1_r4,
  "follows"."updated_at"      AS t1_r5
FROM
  "lists"
LEFT OUTER JOIN
  "follows"
ON
  "follows"."followable_id" = "lists"."id" AND
  "follows"."followable_type" = 'List'
WHERE
  "lists"."is_featured" = 't' AND
  "follows"."user_id" = 1

如何从Rails LEFT OUTER JOIN访问连接记录

1 个答案: