根据Friends Table中的id从User表中检索数据

Question

我有2个表用户和朋友。

用户表字段（id，用户名，密码，fname，lname，昵称等）

朋友表字段（id，user_id，friend_id）

现在问题是我正在尝试根据朋友表friends_id访问用户数据。

mysql查询是：

SELECT *
FROM Users
inner JOIN Friends
ON Users.ID=Friends.Friend_ID;

但需要所有用户数据。

Answer 1

您正在寻找self referential join：

#app/models/user.rb
class User < ActiveRecord::Base
   has_and_belongs_to_many :friends,
     class_name: "User", 
     join_table: :friends,
     foreign_key: :user_id, 
     association_foreign_key: :friend_id
end

您必须更改friends表格以移除id列（根据has_and_belongs_to_many guidelines）：

它可以让你打电话：

#config/routes.rb
resources :users do
   resources :friends, only: [:create, :destroy] #-> url.com/users/:user_id/friends
end

#app/controllers/friends_controller.rb
class FriendsController < ApplicationController
   before_action :set_user

   def create
      @user.friends << @friend
   end

   def destroy
      @user.friends.delete @friend
   end

   private

   def set_user
      @user   = User.find params[:user_id]
      @friend = User.find params[:id]
   end
end

Answer 2

试试这个：

SELECT U1.id AS UserId, 
       U1.username AS UserName, 
       U1.fname AS UserFName, 
       U1.lname AS UserLName,
       U2.id AS FriendId, 
       U2.username AS FriendUserName, 
       U2.fname AS FriendFName, 
       U2.lname AS FriendLName
FROM Friends F 
INNER JOIN Users U1 ON F.user_id = U1.id
INNER JOIN Users U2 ON F.Friend_ID = U2.id;