Hash arguments, default values and asserting valid keys

hash_arguments.rb

def donate_to_charity(args = {})
  # Set valid options that can be supplied. All others will be cut.
  valid_args = [:user_id, :charity_id, :amount]
  
  # Set default options if none are supplied
  defaults = { :user_id    => nil,
               :charity_id => 1,
               :amount     => 25 }
  
  # Raise ArgumentError if invalid keys are passed
  args.assert_valid_keys(*valid_args)

  # Overwrite defaults with any arguments passed in
  opts = defaults.merge(args)

  # Set local variables to use in method (optional)
  user_id    = opts[:user_id]
  charity_id = opts[:charity_id]
  amount     = opts[:amount]

end