gistfile1.rb

module Countable
  def self.included do
    define_method "#{self.name}s" do
      # ...
    end
  end

  def self.name
    self.class.to_s.sub('FeedItem', '').downcase
  end
end

class DonationFeedItem
  include Countable
end

item = DonationFeedItem.new
item.donations

class CountableTest < MiniTest::Unit::TestCase
  class TestFeedItem
    include Countable
  end

  def test_plural_method
    item = TestFeedItem.new
    assert item.tests
  end
end

class DonationFeedItemTest < MiniTest::UnitTestCase
  def test_countable
    assert_includes DonationFeedItem.constants, Countable
  end
end