# migration file
# assuming two models role.rb and recipe.rb 
 
class CreateRecipesRoles < ActiveRecord::Migration
  def self.up
    create_table :recipes_roles, :id => false do |t| 
      t.references :role
      t.references :recipe
    end
 
    add_index :recipes_roles, :role_id
    add_index :recipes_roles, :recipe_id
  end
 
  def self.down
    drop_table :recipes_roles
  end
end
 
 
# in the recipe model 
has_many :roles
 
# in the roles mode
has_many :recipes
 
# in the views/roles/edit.html.erb
<%= select(:role, :recipe_id, Recipes.all.collect() {|r| [r.name, r.id]} , {}, {:multiple => true, :size => 5}   ) %>