CODEWITHSUNDEEP
ruby-on-railsforeign-keysmany-to-manyrelational-database
mike927
mike927

Reputation: 774

Rails Many_to_many association

I am new to Rails and have a problem. I have three tables: books, authors, books_authors. In my view I would like to display two columns. First should display author's name and second all books that author has written decimal. In my books_authors table are foreign keys belongs to primary keys in books and authors tables. My schema:

ActiveRecord::Schema.define(version: 20150709110928) do

create_table "authors", force: :cascade do |t|
  t.string   "name"
  t.string   "surname"
  t.datetime "created_at", null: false
  t.datetime "updated_at", null: false
end

create_table "authors_books", id: false, force: :cascade do |t|
  t.integer  "author_id"
  t.integer  "book_id"
  t.datetime "created_at", null: false
  t.datetime "updated_at", null: false
end

add_index "authors_books", ["author_id"], name:
  "index_author_books_on_author_id"
add_index "authors_books", ["book_id"], name: 
  "index_author_books_on_book_id"

create_table "books", force: :cascade do |t|
  t.string   "title"
  t.datetime "created_at", null: false
  t.datetime "updated_at", null: false
end

and models looks like:

class Author < ActiveRecord::Base
  has_many :authors_books
  has_many :books, through: :authors_books
end

class Book < ActiveRecord::Base
  has_many :authors_books
  has_many :authors, through: :authors_books
end

How could I do it?

Upvotes: 0

Views: 75

Answers (3)

Venkatesh Dhanasekaran
Venkatesh Dhanasekaran

Reputation: 344

Many-to-Many association in rails can be achieve through,
has_many:_______ ; through:______

Ex: Physicians has many patients through appointments.Also, physicians has many appointments.
    Patients has many physicians through appointments. Also, here patients has many appointments.
Here the common entity is appointments. So DHH, has implemented many-to-many like this way.

physicians 
  has_many :appointments
  has_many :patients, through: :appointments
patients
  has_many :appointments
  has_many :physicians, through: :appointments 
appointments
   belongs_to :physicians
   belongs_to :patients

Hope it will helps.

Upvotes: 0

johnnyhappy365
johnnyhappy365

Reputation: 31

I think you should modify the code a little bit. There are two ways I knew to implement many-to-many association:

  1. has_and_belongs_to_many
  2. has_many through

If you want use intermediate join table 'authors_books', you should use has_and_belongs_to_many, but in this case you cannot access authors_books table by model because there is not model about it.

If you want to store some data or info into the intermedidate join table, you should create the model by rails generator cmd like $ rails g model AuthorBook somedata:integer, and use has_many through. At last, delete the 'authors_books' table. The code is as follows:

class Author < ActiveRecord::Base
  has_many :authorbooks
  has_many :books, through: :authorbooks
end

class Book < ActiveRecord::Base
  has_many :authorbooks
  has_many :authors, through: :authorbooks
end

class Authorbook < ActiveRecord::Base
  belongs_to :books
  belongs_to :authors
end

Upvotes: 1

Athar
Athar

Reputation: 3268

In model authors_book, you should do this

class AuthorsBook < ActiveRecord::Base
   belongs_to :author
   belongs_to :book
end

Each entry in authors_books table has author_id and books_id which holds this many to many assosiation. now when you do something this 

  @author.books

it will fetch all the books which that author has written.

You can easily traverse those books and display them.

Upvotes: 1

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