Question

So I’m creating a plugin for Ruby on Rails to make implemented addresses including country, state, city, and zip_code for countries that can follow that paradigm a lot easier but that’s beside the point expect for how the address model is associated.

So starting with my address model.

class Address < ActiveRecord::Base
    has_one :country
    has_one :state
    has_one :city
    has_one :zip_code
end

What’s the difference between saying belongs_to and has_one

Seems to be the same thing because both only require one model to declare ownership and foreign_key

And it also seems that both are logical to say.

an address belongs to an account and an account has one address

Is this only semantics or is there are real difference

Answer 1

From ActiveRecord association’s perspective, any model with belongs_to declaration has the foreign key and any model with has_one declaration has the primary key.

If we go by the current state of your model then you have to ensure tables such as countries, sates, cities, and zip_codes have a column called address_id. Presumably that’s not what you want.

So you have to change your Address model as follows:

class Address < ActiveRecord::Base
  belongs_to :country
  belongs_to :state
  belongs_to :city
  belongs_to :zip_code
end

Which also means you have to ensure addresses table has the following columns: country_id, state_id, city_id and zip_code_id ( I am assuming this is your current table structure).

Edit [I extended my answer to address the questions raised in the comments section.]
In your example you should use has_many association rather has_one association.

class Country
  has_many   :addresses
end

class State
  has_many   :addresses
end

class City
  has_many   :addresses
end

class ZipCode
  has_many   :addresses
end

You can make calls such as:

country.addresses
state.addresses
city.addresses
zip_code.addresses

I have used has_one association in following scenarios.

1) User with a profile

 User has one Profile
 Profile belongs to User

2) Merchant account with a store

 Merchant has one Store     
 Store belongs to Merchant