Convert a belongs_to Association to a has_many :through Association in Ruby on Rails

I was tasked with converting a belongs_to association to a has_many :through association. The challenge was that the app was live, and there were existing relationships. I was able to convert the belongs_to association to a has_many :through association while still maintaining existing relationships by creating a custom migration file, and updating the existing models.

In order to understand how to create this conversion, let’s create a sample app.

If you want to skip ahead to the solution click here.

1. Create a Sample Rails App

Run the following commands in a new terminal window.

rails new association-converter --database=postgresql
cd association-converter/
rails g scaffold Author name:string
rails g scaffold Book name:string author:references
rails db:migrate

2. Create the belongs_to Association

Update the Book and Author models so they are associated with a belongs_to association.

class Book < ApplicationRecord
  belongs_to :author
end

class Author < ApplicationRecord
  has_many :books, dependent: :destroy
end

3. Seed the database

Install Faker and seed the database with sample data.

1. Add gem 'faker', '~> 1.9', '>= 1.9.1' to your Gemfile
2. Run bundle install

3. Open seeds.db and add the following:

   100.times { |i| Author.create(name: Faker::Name.name) }
      
   Author.all.each { |author| author.books.create(name: Faker::Book.title) }
   

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4. Seed the database by running rails db:seed 5. Open up the Rails Console by running rails c. Confirm there is data in the database by running the following:

   2.3.1 :001 > Author.count
   (0.6ms) SELECT COUNT() FROM "authors"
   => 100
   2.3.1 :002 >
   

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   2.3.1 :002 > Book.count
   (14.2ms) SELECT COUNT() FROM "books"
   => 100
   2.3.1 :003 >
   

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   Book.last.author
   Book Load (64.1ms) SELECT "books"._ FROM "books" ORDER BY "books"."id" DESC LIMIT ? [["LIMIT", 1]]
   Author Load (0.3ms) SELECT "authors"._ FROM "authors" WHERE "authors"."id" = ? LIMIT ? [["id", 100], ["LIMIT", 1]]
   => #<Author id: 100, name: "Shirlee Mayert DDS", created_at: "2018-11-21 22:22:46", updated_at: "2018-11-21 22:22:46">
   

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5. Open up schema.rb to confirm the schema looks similar to the following:

   ActiveRecord::Schema.define(version: 2018_11_22_151441) do
     # These are extensions that must be enabled in order to support this database
      
     enable_extension "plpgsql"
      
     create_table "authors", force: :cascade do |t|
       t.string "name"
       t.datetime "created_at", null: false
       t.datetime "updated_at", null: false
     end
      
     create_table "books", force: :cascade do |t|
       t.string "name"
       t.bigint "author_id"
       t.datetime "created_at", null: false
       t.datetime "updated_at", null: false
       t.index ["author_id"], name: "index_books_on_author_id"
     end
      
     add_foreign_key "books", "authors"
   end
   

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At this point we have set up a simple belongs_to association. This will be the baseline to understanding how to convert a belongs_to association to a has_many :through association.

4. Create a Join Table

1. Open a new terminal and run rails g model BooksAuthors book:references author:references.

   The new migration file should render something similar to the following:

   class CreateBooksAuthors < ActiveRecord::Migration[5.2]
     def change
       create_table :books_authors do |t|
         t.references :book, foreign_key: true
         t.references :author, foreign_key: true
       end
     end
   end
   

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2. Update the migration file to use def up and def down methods. Specifically, add the following:

   • In the def up method use create_table to create a new join table.
   • In the def up method populate the new join table with the existing relationships.
   • In the def up method remove the existing reference column.
   • In the def down method add a reference column.
   • In the def down method populate the reference column.
   • In the def down method use the drop_table method to remove the join table.

   class CreateBooksAuthors < ActiveRecord::Migration[5.2]
     def up # create join table
       create_table :books_authors do |t|
         t.references :book, foreign_key: true
         t.references :author, foreign_key: true
      
         t.timestamps
       end
       # populate join table with existing data
       puts "populating books_authors"
       Book.all.each do |book|
         puts "#{book.name} is being added to the books_authors table"
         BooksAuthor.create(book_id: book.id, author_id: book.author_id)
         puts "There are #{BooksAuthor.count} books_authors records"
       end
       # remove obsolete column
       puts "removing old association"
       remove_reference :books, :author, foreign_key: true
     end
      
     def down # add reference column back
       add_reference :books, :author, foreign_key: true # Using a model after changing its table # https://api.rubyonrails.org/classes/ActiveRecord/Migration.html#class-ActiveRecord::Migration-label-Using+a+model+after+changing+its+table
       Book.reset_column_information # associate book with author, even though it will just be one.
       BooksAuthor.all.each do |book_author|
         Book.find(book_author.book_id).update_attribute(
           :author_id,
           book_author.author_id,
         )
       end # remove join table
       drop_table :books_authors
     end
   end
   

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3. Update the database by running rails db:migrate in a terminal window.

4. Open schema.rb to confirm the schema looks similar to the following:

   ActiveRecord::Schema.define(version: 2018_11_22_151828) do
     # These are extensions that must be enabled in order to support this database
      
     enable_extension "plpgsql"
      
     create_table "authors", force: :cascade do |t|
       t.string "name"
       t.datetime "created_at", null: false
       t.datetime "updated_at", null: false
     end
      
     create_table "books", force: :cascade do |t|
       t.string "name"
       t.datetime "created_at", null: false
       t.datetime "updated_at", null: false
     end
      
     create_table "books_authors", force: :cascade do |t|
       t.bigint "book_id"
       t.bigint "author_id"
       t.datetime "created_at", null: false
       t.datetime "updated_at", null: false
       t.index ["author_id"], name: "index_books_authors_on_author_id"
       t.index ["book_id"], name: "index_books_authors_on_book_id"
     end
      
     add_foreign_key "books_authors", "authors"
     add_foreign_key "books_authors", "books"
   end
   

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5. Open the Rails Console by running rails c. Enter the following to confirm the data was migrated correctly:

   2.3.1 :001 > BooksAuthor.count
   (0.1ms) SELECT COUNT(\*) FROM "books_authors"
   => 100
   

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6. Update the existing models.

   • Update books_author.rb

     class BooksAuthor < ApplicationRecord
       belongs_to :book
       belongs_to :author
     end
     

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   • Update book.rb

     class Book < ApplicationRecord
       has_many :books_authors
       has_many :authors, through: :books_authors, dependent: :destroy
     end
     

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   • Update author.rb

     class Author < ApplicationRecord
       has_many :books_authors
       has_many :books, through: :books_authors, dependent: :destroy
     end
     

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7. Open a terminal window and run rails c. Then run the following:

   2.3.1 :003 > Book.last.authors
   Book Load (0.4ms) SELECT "books"._ FROM "books" ORDER BY "books"."id" DESC LIMIT $1 [["LIMIT", 1]]
   Author Load (0.6ms) SELECT "authors"._ FROM "authors" INNER JOIN "books_authors" ON "authors"."id" = "books_authors"."author_id" WHERE "books_authors"."book_id" = $1 LIMIT $2 [["book_id", 100], ["LIMIT", 11]]
   => #<ActiveRecord::Associations::CollectionProxy [#<Author id: 100, name: "Claude O'Keefe", created_at: "2018-11-22 15:16:51", updated_at: "2018-11-22 15:16:51">]>
   

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5. Rolling Back The Conversion

If you need to rollback the conversion, follow these steps.

1. Open up a terminal window and run rails db:rollback.

2. The schema.rb should look similar to the following:

   ActiveRecord::Schema.define(version: 2018_11_22_151441) do
     # These are extensions that must be enabled in order to support this database
      
     enable_extension "plpgsql"
      
     create_table "authors", force: :cascade do |t|
       t.string "name"
       t.datetime "created_at", null: false
       t.datetime "updated_at", null: false
     end
      
     create_table "books", force: :cascade do |t|
       t.string "name"
       t.datetime "created_at", null: false
       t.datetime "updated_at", null: false
       t.bigint "author_id"
       t.index ["author_id"], name: "index_books_on_author_id"
     end
      
     add_foreign_key "books", "authors"
   end
   

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3. Open up a terminal and run rails c. Then run the following:

   2.3.1 :001 > Book.last
   Book Load (0.5ms) SELECT "books".\* FROM "books" ORDER BY "books"."id" DESC LIMIT $1 [["LIMIT", 1]]
   => #<Book id: 100, name: "To Sail Beyond the Sunset", created_at: "2018-11-22 15:16:52", updated_at: "2018-11-22 15:26:07", author_id: 100>
   ```4. Finally, make sure to remove the **join table** model, and revert the **Book** and **Author** models back to a **belongs_to** association.
   

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