models.rb

# MIGRATION

class CreateBasicTables < ActiveRecord::Migration
  def self.up
    create_table :books, :force => true do |t|
      t.string :title
      t.timestamps
    end
    create_table :authors, :force => true do |t|
      t.string :name
      t.timestamps
    end
    create_table :roles, :force => true do |t|
      t.integer :author_id
      t.integer :book_id
      t.string :role_type
      t.timestamps
    end
  end

  def self.down
    drop_table :roles
    drop_table :authors
    drop_table :books
  end
end

# MODELS

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

class Book < ActiveRecord::Base
  has_many :authors, :through => :roles
  has_many :roles
end

class Author < ActiveRecord::Base
  has_many :books, :through => :roles
  has_many :roles
end

# USAGE

b = Book.create(:title => "Beginning FooBar Programming")
a = Author.create(:name => "John SMITH")
r = Role.create(:book => b, :author => a, :role_type => "MAIN_AUTHOR")

b.authors