延續上一篇問題

建立滿足以下關係的Table Associations

  1. User can create a tag for a book.
  2. According to a given tag, return book list.
  3. According to a given book, return tag list.

這次只用一個Join Table但包含了3個FK來實作

建立4個Table

schema
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create_table "books", force: :cascade do |t|
  t.string "title"
end

create_table "tags", force: :cascade do |t|
  t.text "label"
end

create_table "user_book_tags", force: :cascade do |t|
  t.integer "user_id"
  t.integer "book_id"
  t.integer "tag_id"
end

create_table "users", force: :cascade do |t|
  t.string "name"
end

其中user_book_tags包含了user_id, book_id, tag_id，3個Foreign key

設定Model Associations

User
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class User < ActiveRecord::Base
  has_many :user_book_tags
  has_many :tags, through: :user_book_tags
end
Book
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class Book < ActiveRecord::Base
  has_many :user_book_tags
  has_many :tags, through: :user_book_tags
end
Tag
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class Tag < ActiveRecord::Base
  has_many :user_book_tags
  has_many :users, through: :user_book_tags

  has_many :books, through: :user_book_tags
end
UserBookTag
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class UserBookTag < ActiveRecord::Base
  belongs_to :user
  belongs_to :book
  belongs_to :tag
end

再來是在Rails Console裡測試

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# create data
> User.create(name: "Tomo")
> Book.create(title: "Ruby")
> Tag.create(label: "good book")
> UserBookTag.create(user: User.first, book: Book.first, tag: Tag.first)

# test associations
> User.first.tags
> Book.first.tags
> Tag.first.users
> Tag.first.books

與上一次的方法相比，少建立一個Table。