refine_hash.rb

module HashExtensions
  refine Hash do
    # Safely invert
    def invert
      self.reduce({}) do |hash,(key,value)|
        hash.merge!(
          value => (hash[value] || []) + [key]
        )
      end
    end
  end
end

class MyApp
  using HashExtensions

  def self.foo(h)
    h.invert
  end
end

h = {a: 1, b: 1, c: 2}

p h.invert     #=> {1=>:b, 2=>:c}
p MyApp.foo(h) #=> 1=>[:a, :b], 2=>[:c]}