List Attribute Helper

Usage:
class MyModel
  include ListAttributeHelper
  KINDS = %w( stripe paypal )
  scopes_and_identities :kind, KINDS
end
MyModel.paypal.size
=> 999
MyModel.stripe.last.stripe?
=> true
MyModel.stripe.last.paypal?
=> false

module ListAttributeHelper
  module ClassMethods
    def scopes_and_identities(attr_name, arr)
      class_eval do
        arr.each do |val|
          scope val.to_sym, -> { where("#{attr_name} = ?", val) }
          define_method("#{val}?".to_sym) { read_attribute(attr_name.to_sym) == val }
        end
      end
    end
  end

  def self.included(base)
    base.extend(ClassMethods)
  end
end