hash_apply_schema.rb

class Hash

  def apply_schema(schema)
    new_hash = {}
    schema.each do |el|
      if Hash === el
        el.each do |k,v|
          raise ArgumentError.new("Bad schema on key: #{k}, value: #{v}. Value have to be an array") unless Array === v
          new_hash[k] = []
          next unless self.has_key?(k)
          v.each do |vv|
            if Array === vv
              new_hash[k] = self[k].map{|sel| sel.apply_schema(vv)}
            else
              new_hash[k] = self[k].apply_schema(v)
            end
          end
        end
      elsif self.has_key?(el)
        new_hash[el] = self[el]
      else
        new_hash[el] = nil
      end
    end
    new_hash
  end

end



  # Test schema
  schema = [
            :aaa,
            :bbb,
            :ooo,
            {:ccc => [:ddd, :ggg]},
            {:zzz => [[:hhh, :kkk, :mmm]]},
           ]

  input = {
    :aaa => "22",
    :aaaa => "New 22",
    :bbb => "434",
    :ccc => {:ddd   => "abc",
             :ddddd => "Not needed",
             :ggg   => "Needed",
             :hard_to_believe => []},
    :zzz => [
      {:hhh  => 126,
       :hhhh => "Don't need",
       :kkk  => "Existing key"},
      {:hhh  => "DoobyDo",
       :kkk  => "Needed",
       :mmm  => "Existing key"}
    ]
  }

  output = {
    :aaa => "22",
    :bbb => "434",
    :ooo => nil,
    :ccc => {:ddd => "abc", :ggg => "Needed"},
    :zzz => [{:hhh => 126, :kkk => "Existing key", :mmm => nil},
             {:hhh=>"DoobyDo", :kkk=>"Needed", :mmm=>"Existing key"}]
  }

  puts
  puts "schema:\n    #{schema}"
  puts "input:\n    #{input}"
  puts "desired_output:\n   #{output}"
  puts "apply:\n    #{input.apply_schema(schema)}"