join - 三向关联查询/has_many :through/join

Question

我正在使用可用的指南/api/书籍自学 Rails，但我无法理解通过三种方式/嵌套 has_many :through 关联进行的连接。

我有用户与组相关联:通过成员(member)资格。

我在多对多的组中也有帖子。同一个帖子可以发布到多个组 + 组可以有很多帖子。

我希望能够为用户的主页做的是，显示组的所有distinct posts < strong>用户是的成员。

例如。 current_user.groups.posts # 我希望这很简单!!

这是我的代码。

Models:

class User < ActiveRecord::Base
  has_many :memberships
  has_many :groups, :through => :memberships
  has_many :posts  # as author of post
end

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

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

class Post < ActiveRecord::Base
  belongs_to :user
  has_and_belongs_to_many :groups
end

路线.rb

Myapp::Application.routes.draw do
  get "admin/index"
  devise_for :users

  resources :users do
    member do
      get :groups
    end
  end

  resources :groups do
    member do
      get :members
      post :join
      post :leave
    end
  end

  resources :posts

home_controller.rb#index

class HomeController < ApplicationController
  before_filter :authenticate_user!

  def index
    @user = current_user
    @groups = Group.all
    @user_groups = @user.groups
    @home_page_posts = Post.joins(:groups, :user)
  end
end

这显然只是为我提供了所有组中所有帖子的不明确列表。

如果有人能指出我正确的方向。我试过了 http://guides.rubyonrails.org/active_record_querying.html#joining-tables但据我所知，这些例子都不适用。

如果您需要我提供更多信息，请告诉我。 :D

Answer 1

我相信这就是你想要的:

Post.includes(:groups => :users).where('users.id' => current_user.id)

这将生成这样的 SQL(对于 current_user.id = 1):

SELECT "posts"."id" AS t0_r0, "posts"."user_id" AS t0_r1,
  "posts"."created_at" AS t0_r2, "posts"."updated_at" AS t0_r3,
  "groups"."id" AS t1_r0, "groups"."created_at" AS t1_r1,
  "groups"."updated_at" AS t1_r2, "users"."id" AS t2_r0,
  "users"."name" AS t2_r1, "users"."created_at" AS t2_r2,
  "users"."updated_at" AS t2_r3
  FROM "posts"
  LEFT OUTER JOIN "groups_posts" ON "groups_posts"."post_id" = "posts"."id"
  LEFT OUTER JOIN "groups" ON "groups"."id" = "groups_posts"."group_id"
  LEFT OUTER JOIN "memberships" ON "memberships"."group_id" = "groups"."id"
  LEFT OUTER JOIN "users" ON "users"."id" = "memberships"."user_id"
  WHERE "users"."id" = 1

另请参阅:Filter model with multiple has many through (不完全相同，但相似)