AddProjectUserJoinTable.rb

class CreateProducts < ActiveRecord::Migration
  def up
    create_table :projects_users, :id => false do |t|
      t.references :user, :null => false
      t.references :project, :null => false
    end
  end
  def down
   drop_table :projects_users
  end
end

project.rb

class Project < ActiveRecord::Base
  attr_accessible :description, :name
  
  has_and_belongs_to_many :users
end

user.rb

class User < ActiveRecord::Base
  attr_accessible :email, :password, :password_confirmation
  has_secure_password
  validates_uniqueness_of :email
  
  has_and_belongs_to_many :projects
end