Question

我有一个包含用户，图书馆和图书的应用。

用户拥有许多库。图书馆有很多用户和书籍。但是用户和书籍没有直接的关系。用户只能从图书馆借阅图书。

我试图获取属于用户所属图书馆的图书集。换句话说，我希望获得给定用户所属的库中的所有书籍。

要说清楚，我不想通过目前存在于用户和图书之间的借用协会，因为我想要该图书馆中的所有图书，而不仅仅是当前用户借阅的图书。

我已经尝试使用SQL和Squeel在几个小时内提出这个查询，没有运气......在此先感谢您的帮助。

Answer 1

你可以这样做......

Book.where(:library_id => user.library_ids)

或者你可以这样做。鉴于此模型设置...

class Book < ActiveRecord::Base
  belongs_to :library

class Library < ActiveRecord::Base
  has_many :books
  has_many :memberships
  has_many :users, :through => :memberships

class User < ActiveRecord::Base
  has_many :memberships
  has_many :libraries, :through => :memberships
  has_many :books, :through => :libraries

有了这个，你可以要求@user.books，它在Rails中会产生如下的SQL：

# 1 is @user.id
SELECT "books".* FROM "books" INNER JOIN "libraries" ON "books"."library_id" = "libraries"."id" INNER JOIN "memberships" ON "libraries"."id" = "memberships"."library_id" WHERE "memberships"."user_id" = 1

Answer 2

Table_User
user_id user_name library_id membership_id

Table_Library
library_id user_id book_id membership_id

Table_Books
books_id book_name library_id


Table_Membership

membership_id user_id library_id

select book_name from Table_Books
inner join Table_Library on Table_Books.library_id=Table_Library.library_id 
inner join Table_User on Table_User.library_id=Table_Library.library_id
where Table_User.library_id = Table_Membership.library_id 
and Table_User.user_id =Table_Membership.user_id

查询间接关系

2 个答案: