ruby-on-rails - 如何(通过关系表)加入针对自身的表？

Question

考虑下表:

人:

id: integer
name: string

friend :

id1: integer
id2: integer

我想找到所有的 friend 对，所以我使用这样的东西:

People.joins("JOIN friends ON friend.id1 = people.id JOIN friends ON friend.id2 = people.id")

生成此 SQL:

SELECT "people".* FROM "people" 
JOIN friends ON friends.id1 = people.id 
JOIN people ON friends.id2 = people.id

并出现以下错误:

(pry) output error: #<ActiveRecord::StatementInvalid:
PG::DuplicateAlias: ERROR:  table name "people" specified more than once

多次指定 people 有什么问题？

Answer 1

给他们不同的别名:

SELECT p1.* FROM people p1
JOIN friends ON friends.id1 = p1.id 
JOIN people p2 ON friends.id2 = p2.id