Question

据我所知，Rails执行LEFT JOIN的唯一方法是手动编写SQL，如下所示：

Company.joins('LEFT JOIN people ON companies.id = people.company_id')

但是，加入其他表并使用.includes(:people)变得非常棘手。

我需要使用.includes(:people)，因为每当我访问people方法时，Rails都会生成额外的数据库请求（相关问题：When joining table, rails anyway makes additional request when accessing fields from joined table）

所以，这有效：

Company.joins('LEFT JOIN people ON companies.id = people.company_id')

这有效：

Company.joins('LEFT JOIN people ON companies.id = people.company_id')
      .includes(:people)

这有效：

Company.joins('LEFT JOIN people ON companies.id = people.company_id')
      .includes(:people).joins(:city)

不起作用：

Company.joins('LEFT JOIN people ON companies.id = people.company_id')
      .includes(:people).joins(:city).includes(:city)

最终加入people两次，当然db抱怨模糊列名称：people.id。

生成的SQL是：

SELECT "companies"."id" AS t0_r0, "companies"."title" AS t0_r1, 
"companies"."address" AS t0_r2, "companies"."city_id" AS t0_r3, 
"companies"."created_at" AS t0_r4, "companies"."updated_at" AS t0_r5, 
"people"."id" AS t1_r0, "people"."name" AS t1_r1, "people"."surname" AS t1_r2,
"people"."patronymic" AS t1_r3, "people"."company_id" AS t1_r4,
"people"."created_at" AS t1_r5, "people"."updated_at" AS t1_r6,
"cities"."id" AS t2_r0, "cities"."title" AS t2_r1, "cities"."created_at" AS t2_r2,
"cities"."updated_at" AS t2_r3, "cities"."test" AS t2_r4 FROM "companies" 
INNER JOIN "cities" ON "cities"."id" = "companies"."city_id"
LEFT OUTER JOIN "people" ON "people"."company_id" = "companies"."id"
LEFT JOIN people ON companies.id = people.company_id

我的理解是，当我们手动编写JOIN SQL代码时，Rails无法控制它，并且它无法确定people表已经加入。

如果我这样写，它就有效：

Company.joins(:people).includes(:people).joins(:city).includes(:city)

但是，它使用INNER JOIN，但我需要LEFT JOIN。我仍然无法找到使用LEFT JOIN的方法，以便在Rails的控制之下。

怎么做？

Answer 1

知道了：我们需要将.references()与.includes结合使用;然后，Rails执行LEFT JOIN，我们可以在SQL查询中引用连接表。

所以，这有效：

Company.includes(:people).references(:people).includes(:city).references(:city)

嗯，这看起来很混乱。

左连接和.includes（）

1 个答案: