Set model relationship using key instead of object

attribute_key_writer.rb

module AttributeKeyWriter
  extend ActiveSupport::Concern

  module ClassMethods
    protected
      # Allow writing an attribute by key/code
      #
      # Example:
      #   belongs_to :currency
      #   attr_key_writer(:currency, key: "code")
      #
      # And now you can set the value with either object or code
      # user.currency = Currency.find(1)
      # user.currency = "USD"
      def attr_key_writer(*attributes)
        options = attributes.extract_options!
        key = options[:key] || :code

        attributes.each do |attribute|
          define_method("#{attribute}=") do |value|
            super(value.is_a?(String) ? attribute.to_s.classify.constantize.where(key => value).first : value)
          end
        end
      end
  end
end