左连接和.includes()

Question

据我了解，Rails 中执行

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)

也变得非常棘手。

我需要使用

.includes(:people)

，否则Rails会在我访问

people

方法时生成额外的数据库请求（相关问题：当连接表时，rails在访问连接表中的字段时无论如何都会发出额外的请求）

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)

这不起作用：

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

最终会加入

people

两次，当然数据库会抱怨列名不明确：

people.id

。

生成的SQL为：

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，它无法弄清楚

people

表已经加入。

如果我这样写，它就可以工作：

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

但是，它使用

INNER JOIN

，但我需要

LEFT JOIN

。我仍然找不到使用

LEFT JOIN

的方法，以便这处于 Rails 的控制之下。

如何做到这一点？

Answer 1

明白了：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。

Answer 2

那

Company.joins(:people, :city).select(*)

呢？