1:N and N:N Associations

Setup

For this lab you'll need to start and setup a new application.

• rails new assoc_lab -d postgresql
• cd assoc_lab
• rake db:create (Why?)

1:N

The one-to-many relationship is the most common relationship one will come across when developing an application

Authors Table

id	first_name	last_name	y_o_b	y_o_d
1	Rudyard	Kipling	1865	1936
2	Lewis	Carroll	1832	1892
3	H.G.	Wells	1866	1946

Books Table

id	title	description	publication_year	isbn	author_id
1	The Jungle Book	The Jungle Book is a collection of stories by English author Rudyard Kipling. The stories were first published in magazines in 1893–94. The original publications contain illustrations, some by Rudyard's father, John Lockwood Kipling.	1894	9788497896696	1
2	Alice's Adventures in Wonderland	Alice's Adventures in Wonderland is an 1865 novel written by English author Charles Lutwidge Dodgson under the pseudonym Lewis Carroll.	1865	9781552465707	2
3	Rikki-Tikki-Tavi	"Rikki-Tikki-Tavi" is a short story in The Jungle Book by Rudyard Kipling about the adventures of a valiant young mongoose. The story has often been anthologized, and has been published more than once as a short book in its own right.	1894	1484123689	1
4	Through the Looking-Glass	Through the Looking-Glass, and What Alice Found There is a novel by Lewis Carroll, the sequel to Alice's Adventures in Wonderland. It is based on his meeting with another Alice, Alice Raikes	1871	9781489500182	2
5	The Time Machine	The Time Machine is a science fiction novel by H. G. Wells, published in 1895. Wells is generally credited with the popularisation of the concept of time travel by using a vehicle that allows an operator to travel purposefully and selectively	1895	9781423794417	3

Explanation

In this example we have shown how to associate tables using the concept of a foriegn_key. In the above Books table the author_id is a foriegn key that we can use to look up information about the author.

Implementation

We want to generate these models and setup the associations. First let's setup the Author model.

• Note that in the following y_o_b and y_o_d could be called yob and yod to cut down on typing.

   rails g model author first_name:string last_name:string y_o_b:integer y_o_d:integer
  

• Now let's generate the Book model. Keep in mind that here we will need a column for the foriegn key to associate to the Author model.

   rails g model book title:string description:text publication_year:integer isbn:string author_id:integer
  

• Always be sure to migrate your changes

   rake db:migrate
  

• Setup the associations to books for author

   class Author < ActiveRecord::Base
   	has_many :books
   end
  

• Setup the associations to author for book

   class Author < ActiveRecord::Base
   	belongs_to :author
   end
  

Working with Active Record in the Command Line

• Open the Rails console

• First create some authors. Please use the ones listed in the table above for the sake of consistency.

   > Author.create(first_name:"Rudyard", last_name:"Kipling")
  

• Find Rudyard Kipling like so (only find by id if you know for sure what the id is):

   > rud = Author.find(1)
   or
   > rud = Author.find_by(first_name:"Rudyard")
  

• Add The Jungle Book as a book to rud using the helper methods, i.e.

   > rud.books.create(title: "The Jungle Book", description: "Cool sh!t", publication_year: 1894, isbn: 9788497896696)
  

• Find The Jungle Book and update its description.

   > jungle_bk = rud.books.find_by(isbn: 9788497896696)
   > jungle_bk.update(description: "The sh!t.")
  

• Or we can do the following, which is just the typical way to update. However, keep in mind that this doesn't guarantee that the book we are updating is properly associated to our author, good ole rud.

  > jungle_bk = Book.find_by(isbn: 9788497896696)
  > jungle_bk.update(description: "very cool")
  

• Let's create a second book for Rikki-Tikki_tavi using a different way.

  > rikki_tikki = Book.create(title: "Rikki-Tikki-Tavi", isbn: 1484123689)
  > rud.books.push(rikki_tikki)
  

• Verify that rud is associated to the new book.

  > rud.books.find(isbn: 1484123689) 
  # or do the following
  > rikki_tikki = Book.find_by(isbn: 1484123689)
  > rikki_tikki.author_id == rud.id # should be true
  

Moar

Repeat the process we went through for rud above with lewis_carroll and h_g_wells.

N:N

The many to many (N:N) relationship is about as common of an association as 1:N.

Implementation

A very common form of association for the many-to-many association is the library-book association. A many-to-many association will need what's called a join table that will store the foriegn keys of the table rows being associated. I call it the glue table.

Books Table

id	title	description	publication_year	isbn	author_id
1	The Jungle Book	The Jungle Book is a collection of stories by English author Rudyard Kipling. The stories were first published in magazines in 1893–94. The original publications contain illustrations, some by Rudyard's father, John Lockwood Kipling.	1894	9788497896696	1
2	Alice's Adventures in Wonderland	Alice's Adventures in Wonderland is an 1865 novel written by English author Charles Lutwidge Dodgson under the pseudonym Lewis Carroll.	1865	9781552465707	2
3	Rikki-Tikki-Tavi	"Rikki-Tikki-Tavi" is a short story in The Jungle Book by Rudyard Kipling about the adventures of a valiant young mongoose. The story has often been anthologized, and has been published more than once as a short book in its own right.	1894	1484123689	1
4	Through the Looking-Glass	Through the Looking-Glass, and What Alice Found There is a novel by Lewis Carroll, the sequel to Alice's Adventures in Wonderland. It is based on his meeting with another Alice, Alice Raikes	1871	9781489500182	2
5	The Time Machine	The Time Machine is a science fiction novel by H. G. Wells, published in 1895. Wells is generally credited with the popularisation of the concept of time travel by using a vehicle that allows an operator to travel purposefully and selectively	1895	9781423794417	3

BooksLibraries Table

id	library_id	book_id
1	1	1
2	1	2
3	1	3
4	2	2
5	2	3
6	3	1
6	3	2

Libraries Table

id	name	city	state
1	Library of Congress	Washington	DC
2	George Peabody Library	Baltimore	Maryland
3	Central Library	Seattle	Washington

Implementation

• Generate a model called BooksLibrary with columns to represent foreign keys

-> Books is pluralized and Library is not because we ultimately want Rails to create a table called BooksLibaries. Rails will pluralize model names in order to generate table names (it's smart enough to change Library to Libraries, but we need to pluralize Books for it)

rails g model BooksLibrary library_id:integer book_id:integer

• Generate a model for the Library

  rails g model Library name:string city:string state:string
  

• Make sure to migrate

rake db:migrate

• Set up the association between the BooksLibrary model and Book model

  class BooksLibrary < ActiveRecord::Base
    belongs_to :book
  end
  

• Set up the association between the BooksLibrary model and Library model

  class BooksLibrary < ActiveRecord::Base
    belongs_to :book
    belongs_to :library
  end
  

• Set up the association between the Book model and BooksLibrary model

  class Book < ActiveRecord::Base
    belongs_to :author
    has_many :books_libraries
  end
  

• Set up the association between the Book model and the Library model through the join table model

  class Book < ActiveRecord::Base
    belongs_to :author
    has_many :books_libraries
    has_many :libraries, through: :books_libraries
  end
  

• Set up the association between the Library model and the BooksLibrary model.

  class Library < ActiveRecord::Base
    has_many :books_libraries
  end
  

• Setup the association between the Library model and the Book model through the join table model.

  class Library < ActiveRecord::Base
    has_many :books_libraries
    has_many :books, through: :books_libraries
  end
  

Back to the Command Line

• Create some libraries using the table above as reference

• Add an associated book to a library in rails console.

    > congress = Library.find(1) # library of congress
    > jungle_bk = Book.find_by(isbn: 9788497896696)
    > congress.books.push(jungle_bk)
  

• Add rikki_tikki and the_time_machine to library of congress by finding them and pushing them into the congress.books collection.

• Find the George Peabody Library and add rikki_tikki and the_time_machine to the g_p_lib.books collection using push.

• Find the Central Library and add the jungle_bk and rikki_tikki to the central_lib.books collection using push.

• Type the following in rails console:

  > BooksLibrary.all.count
  

  • How many BooksLibrary objects are there?
  • Why does that make sense?

• Using the congress library try the following:

  > Book.all.count
  > BooksLibrary.all.count
  > congress.books.count
  > congress.books.delete(1)
  > Book.all.count
  > BooksLibrary.count
  

  • Which number changed after the congress.books.delete, Book.all.count or BooksLibrary.all.count?
  • Why does that make sense?

Bonus

Add a User model to your application. A User has many favorite books, and a book can be favorited by many users. Hmm, what kind of relationship is that?