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.

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

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) }

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 >

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

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">

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

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

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

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

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

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

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
```

Update the existing models.

Update books_author.rb

class BooksAuthor < ApplicationRecord
  belongs_to :book
  belongs_to :author
end

Update book.rb

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

Update author.rb

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

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">]>

5. Rolling Back The Conversion

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

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

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

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.