class User < ActiveRecord::Base
  has_one :city
  has_one :state
  has_one :country
end

class City < ActiveRecord::Base
  belongs_to :state
  belongs_to :user
end

class State < ActiveRecord::Base
  has_many :cities
  belongs_to :country
  belongs_to :user
end

class Country < ActiveRecord::Base
  has_many :states
  belongs_to :user
end

city       = City.create(name: "New York City")
city.state = state

state         = State.create(name: "New York")
state.country = country

country = Country.create(name: "United States")

user         = User.create(name: "John Doe")
user.city    = City.first
user.state   = user.city.state
user.country = user.state.country