Using decorators to extend objects with roles without modifying the original object. A possible alternative to dynamic method binding described in Clean Ruby.

gistfile1.rb

# Jim Gay's new book, Clean Ruby (http://clean-ruby.com), has a section on dynamically binding
# methods to objects with roles in a DCI context. He does this in a way that doesn't modify the
# original object, unlike extending the object with a model does.  I found the solution clever
# but maybe a little bit unclear (but it's the first iteration of this part of the book, so I'm
# sure it will become more convincing later :).
#
# This is an alternative to that solution which uses Module extension on a simple wrapper/decorator
# object. The module's methods are added to the wrapper and it behaves as though the original object
# has been extended with the role, but the original is untouched. I haven't really seen this solution
# to the role extension problem anywhere else, but I'm sure I'm not the first to suggest it.

require 'delegate'

module Transferrable
  def transfer_to(amount, to)
    self.decrement(amount)
    to.increment(amount)
  end
end

source = Account.find(1)
destination = Account.find(2)

# This would go in a Context that needs the role.
transferrable_source = SimpleDelegate.new(source).extend(Transferrable)
transferrable_source.transfer_to(10, destination)

source.transfer_to(10, destination) # raises NoMethodError, that's good!