gistfile1.rb

class User
  has_many :city_users
  has_many :cities, :through => :city_users
end

class City
  has_many :city_users
  has_many :users, :through => :city_users
end

class CityUser
  belongs_to :user
  belongs_to :city
end

user = User.new
city = City.new

user.cities << city # add another city to a user
user.city_ids = [1,2,3] # assign a bulk of city id-s directly