class Hash

def as_json(options = nil) #:nodoc:
  # create a subset of the hash by applying :only or :except
  subset = if options
    if attrs = options[:only]
      slice(*Array(attrs))
    elsif attrs = options[:except]
      except(*Array(attrs))
    else
      self
    end
  else
    self
  end
  # use encoder as a proxy to call as_json on all values in the subset, to protect from circular references
  encoder = options && options[:encoder] || ActiveSupport::JSON::Encoding::Encoder.new(options)
  Hash[subset.map { |k, v| [k.to_s, encoder.as_json(v, options)] }]
end