City Validator for Metropoli (github.com/innku/metropoli)

metropoli_city_validator.rb

# lib/validators/metropoli_city_validator.rb
class MetropoliCityValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)  
    record.errors[:base] << 'Ciudad no valida' if Metropoli::CityModel.with_values(value).empty?
  end  
end

some_model.rb

# app/models/some_model.rb
class SomeModel < ActiveRecord::Base

  # ...
  validates :city_name, :presence => true, :metropoli_city => true
  # ...

end