Two ways to do many to many relations in RoR

Many to Many in RoR

# http://blog.hasmanythrough.com/2006/4/20/many-to-many-dance-off

# Join Table: Simple Associations
# Table:

create_table "dancers_movies", :id => false do |t|
  t.column "dancer_id", :integer, :null => false
  t.column "movie_id",  :integer, :null => false
end

# Models:

class Dancer < ActiveRecord::Base
  has_and_belongs_to_many :movies
end

class Movie < ActiveRecord::Base
  has_and_belongs_to_many :dancers
end

# Join Model: Rich Associations
# Table:

create_table "appearances", do |t|
  t.column "dancer_id",      :integer, :null => false
  t.column "movie_id",       :integer, :null => false
  t.column "character_name", :string
  t.column "dance_numbers",  :integer
end

# Models:

class Appearance < ActiveRecord::Base
  belongs_to :dancer
  belongs_to :movie
end

class Dancer < ActiveRecord::Base
  has_many :appearances, :dependent => true
  has_many :movies, :through => :appearances
end

class Movie < ActiveRecord::Base
  has_many :appearances, :dependent => true
  has_many :dancers, :through => :appearances
end