hash_with_underscore_key is only converting key on Hash into underscore.

array.rb

class Array
  def to_underscore_key
    self.class.new.tap do |array|
      self.each { |value|
        case value
        when Hash, Array
          array << value.to_underscore_key
        else
          array << value
        end
      }
    end
  end
end

hash.rb

class Hash
  def to_underscore_key
    self.class.new.tap do |hash|
      self.each_pair { |key, value|
        case key
        when Hash, Array, String, Symbol
          case value
          when Hash, Array
            hash[key.to_underscore_key] = value.to_underscore_key
          else
            hash[key.to_underscore_key] = value
          end
        else
          case value
          when Hash, Array
            hash[key] = value.to_underscore_key
          else
            hash[key] = value
          end
        end
      }
    end
  end
end

string.rb

class String
  def to_underscore_key
    self.underscore.gsub(/([^\d\s]+)(\d)/, "\\1_\\2")
  end
end

symbol.rb

class Symbol
  def to_underscore_key
    self.to_s.to_underscore_key.to_sym
  end
end