Hash flattener for Ruby

flattener.rb

class Flattener
  def initialize(separator='_')
    @separator = separator
  end

  def flatten(hash, prefix=nil)
    Enumerator.new do |yielder|
      hash.each do |key, value|
        raise "Bad key: #{key.inspect}" unless key.is_a?(String)
        key = [prefix, key].join(@separator) if prefix

        if value.is_a?(Hash)
          flatten(value, key).each do |nested_key_value|
            yielder.yield nested_key_value
          end
        else
          yielder.yield [key, value]
        end
      end
    end
  end
end

require 'rspec'

describe Flattener do
  let(:input_hash) do
    {
      'key1' => :value1,
      'nested' => {
        'key2' => :value2,
        'nested2' => {
          'key3'  => :value3
        }
      },
      'key4' => :value4
    }
  end

  let(:flattener) { Flattener.new }
  subject { flattener.flatten(input_hash).to_a }

  specify { expect(subject[0]).to eq(['key1', :value1]) }
  specify { expect(subject[1]).to eq(['nested_key2', :value2]) }
  specify { expect(subject[2]).to eq(['nested_nested2_key3', :value3]) }
  specify { expect(subject[3]).to eq(['key4', :value4]) }
end

require 'pp'

example_hash =  {'key1' => :value1, 'nested' =>{'key2' => :value2, 'nested2' => {'key3'  => :value3}}, 'key4' => :value4}
pp example_hash
pp Hash[Flattener.new.flatten(example_hash).to_a]