hashy.rb

h1 = Hash.new { |h,k| h[k] = Hash.new(&h.default_proc) }.with_indifferent_access
h2 = h1.dup

h1['x']['y']['a'] = 5
h1['x']['y']['arr'] = [1,2,3]
h2['x']['y']['arr'] = [2,3,4]
h1['h'] = [1,2]
h2['f'] = [3,4]

h1.deep_merge!(h2) { |key, old_value, new_value| (Array(old_value) + Array(new_value)).uniq }