Rails - class_name

When trying to establish relationship between 2 models, we have to specify :class_name if 'module' is different

/app/models/abc/address.rb
module ABC
class Address < ActiveRecord::Base
belongs_to :user
                attr_accessible :user_id, :address1, :address2, :city, :country, :state, :zip, :is_primary_address
         end
end

/app/models/user.rb
class User < ActiveRecord::Base
       has_many :addresses, :class_name => ABC::Address
       ...
       ...
end