使用 ActiveRecord 和 Rails 3 进行复杂的 JOIN答案

【问题标题】：Complex JOIN with ActiveRecord and Rails 3使用 ActiveRecord 和 Rails 3 进行复杂的 JOIN
【发布时间】：2019-11-23 01:13:50
【问题描述】：

我有以下型号：

class User < ActiveRecord::Base
  has_many :memberships
  has_many :groups, :through => :memberships
end

class Group < ActiveRecord::Base
  has_many :memberships
  has_many :users, :through => :memberships
end

class Membership < ActiveRecord::Base
  belongs_to :user
  belongs_to :group
end

class Post < ActiveRecord::Base
  belongs_to :group
end

我必须找到属于用户所属组的所有帖子。我用这种方法做到了：

@post = Post
  .joins(:group => {:memberships => :user})
  .where(:memberships => {:user_id => current_user.id})

但它会产生低效的 SQL：

SELECT "posts".* FROM "posts" 
INNER JOIN "groups" ON "groups"."id" = "posts"."group_id" 
INNER JOIN "memberships" ON "memberships"."group_id" = "groups"."id" 
INNER JOIN "users" ON "users"."id" = "memberships"."user_id" 
WHERE "memberships"."user_id" = 1

我想进行这样的查询：

SELECT posts.* FROM posts 
INNER JOIN memberships ON memberships.group_id = posts.group_id 
WHERE memberships.user_id = 1

如何在不使用原始 SQL 的情况下做到这一点？

【问题讨论】：

标签： sql ruby-on-rails ruby-on-rails-3 activerecord join

【解决方案1】：

你可以在不改变你的模型的情况下更接近，通过从你的通话中删除未使用的加入：

Post.joins(group: :memberships).where(memberships: { user_id: 1 })

编译成 SQL

SELECT "posts".* FROM "posts"
INNER JOIN "groups" ON "groups"."id" = "posts"."group_id"
INNER JOIN "memberships" ON "memberships"."group_id" = "groups"."id" 
WHERE ("memberships"."user_id" = 1)

【讨论】：

【解决方案2】：

这样的东西应该适合你，尽管它需要混合一些原始 SQL

Post
  .joins("INNER JOIN memberships ON memberships.group_id = posts.group_id")
  .where(:memberships => {:user_id => current_user.id})

【讨论】：

如何从组表中选择特定列？