example_zipcode.rb

class Company
  def zipcode
    Zipcode.parse(get_attribute(:zipcode), self.country)
  end
end

# example zipcode interface
class Zipcode
  def self.parse(zipcode, country)
    Object.const_get("Zipcode::#{country.upcase}").new(zipcode)
  end
  # the local part of zipcode that is typically included in things
  # subclass impl
  def local
  end
  
  # if supported, additional 4 digits
  def extension
  end
end

some_mid = params[:mid]
zipcode = params[:zipcode]
Company.find_by_mid(some_mid).select{|c| c.zipcode.local.eql?(zipcode)