Question

I am using Ruby on Rails v3.2.2. I have following model classes

class Country < ActiveRecord::Base
  has_many :regions, :foreign_key => 'country_id'
end

class Region < ActiveRecord::Base
  belongs_to :country, :foreign_key => 'country_id'
  has_many :cities, :foreign_key => 'region_id'
end

class City < ActiveRecord::Base
  belongs_to :region, :foreign_key => 'region_id'
end

and I would like to make a City belongs_to :country.

I know that the simplest way to make that is to add a country_id database table column to the City database table and to state related ActiveRecord Associations, this way:

class Country < ActiveRecord::Base
  # ...
  has_many :cities, :foreign_key => 'country_id'
end

class City < ActiveRecord::Base
  # ...
  belongs_to :country, :foreign_key => 'country_id'
end

However, in order to store less database data, I think I may “use” the data already stored in the Region table since a city belongs to a region which in turn belongs to a country (this implies that a city belongs to a country) but, in this case, I have no idea on how to properly state ActiveRecord Associations for City and Country so to “exploit” mentioned relationship informations implicitly present “through” the Region model class.

How should I proceed?

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Note: I am “forcing” to state the belongs_to :country ActiveRecord Association in the City model class because I would like to use the RoR :counter_cache feature (available only for belongs_to associations) in order to count cities present in a country.

Answer 1

Use the :through option. As I’ve seen in your comment in the answer below (which by the way, was correct), you’ll just have to add this:

has_one :country, :through => :region

to your City class. If you want to apply counter_cache on country for the cities, then you’ll have to establish the relationship in the country class as well, like this:

has_many :cities, :through => :regions

and then you can have your count column