hash_filter.rb

require 'yaml'
class Hash
  def filter(*keys)
    self.inject({}) {|h,(k,v)| h[k] = v if keys.include?(k); h}
  end
end

test_hash = {'one' => 1, 'two' => 2, 'three' => 3}
puts test_hash.to_yaml

puts test_hash.filter('one').to_yaml

puts test_hash.filter('one', 'two').to_yaml