What does #self.included(base) do in Ruby on Rails' Restful Authentication?

The self.included function is called when the module is included. It allows methods to be executed in the context of the base (where the module is included).

More info: a ruby mixin tutorial.

Out of the same reason which Peter has mentioned I would like to add an example so that it's easy for the newbie developers to understand self.included(base) and self.extended(base) :

module Module1
 def fun1
    puts "fun1 from Module1"
 end

 def self.included(base)
    def fun2
        puts "fun2 from Module1"
    end
 end

 def self.extended(base)
   def fun3
     puts "fun3 from Module1"
   end
 end

end

module Module2
 def foo
    puts "foo from Module2"
 end

 def self.extended(base)
    def bar
        puts "bar from Module2"
    end
 end
end


class Test
include Module1
extend Module2
 def abc
    puts "abc form Test"
 end
end

class Test2
  extend Module1
end

Test.new.abc #=> abc form Test

Test.new.fun1 #=> fun1 from Module1

Test.new.fun2 #=> fun2 from Module1

Test.foo #=> foo from Module2

Test.bar #=> bar from Module2

Test.new.fun3 #=> NoMethodError (undefined method `fun3' ..)

Test2.fun3 #=> fun3 from Module1

extend : methods will be accessible as class methods

include : methods will be available as instance methods

"base" in self.extended(base) / self.included(base) :

The base parameter in the static extended method will be either an instance object or class object of the class that extended the module depending whether you extend a object or class, respectively.

When a class includes a module the module’s self.included method will be invoked. The base parameter will be a class object for the class that includes the module.

When the AuthenticatedSystem method is included using the include method, the self.included method is triggered with whatever it was included into being the argument of base.

The code you've shown calls helper_method and defines some helpful helpers, but only if the base has a helper_method method.

It's done that way so including the module can set up the helper methods as well as adding additional methods to the class.