avoiding memory leak in Rails 3 development mode (where cache_classes == false); drawback is, that now the state machine won't be re-declared on each request; thus, changes in the state machine will force you to restart the Rails server (but this still is better than the memory leak)

lib/aasm/state_machine.rb

@@ -1,13 +1,12 @@
 class AASM::StateMachine
-  def self.[](*args)
-    (@machines ||= {})[args]
+  def self.[](clazz)
+    (@machines ||= {})[clazz.to_s]
   end
-
-  def self.[]=(*args)
-    val = args.pop
-    (@machines ||= {})[args] = val
+  
+  def self.[]=(clazz, machine)
+    (@machines ||= {})[clazz.to_s] = machine
   end
-
+  
   attr_accessor :states, :events, :initial_state, :config
   attr_reader :name
 

test/models/process.rb

@@ -0,0 +1,18 @@
+module Models
+  class Process
+    include AASM
+
+    aasm_state :sleeping
+    aasm_state :running
+    aasm_state :suspended
+
+    aasm_event :start do
+      transitions :from => :sleeping, :to => :running
+    end
+
+    aasm_event :stop do
+      transitions :from => :running, :to => :suspended
+    end
+
+  end
+end

test/unit/state_machine_test.rb

@@ -0,0 +1,37 @@
+require File.expand_path(File.dirname(__FILE__) + '/../test_helper')
+
+class StateMachineTest < Test::Unit::TestCase
+  
+  context "state machines" do
+    
+    should "be created without memory leak" do
+      assert_equal 1, AASM::StateMachine.instance_variable_get("@machines").size  # AuthMachine      
+      assert_number_of_objects AASM::SupportingClasses::State, 5                  # AuthMachine
+      assert_number_of_objects AASM::SupportingClasses::Event, 10                 # AuthMachine
+      assert_number_of_objects AASM::SupportingClasses::StateTransition, 18       # AuthMachine
+      
+      load File.expand_path(File.dirname(__FILE__) + '/../models/process.rb')
+      assert_equal 2, AASM::StateMachine.instance_variable_get("@machines").size  # AuthMachine + Process
+      assert_number_of_objects Models::Process, 0
+      assert_number_of_objects AASM::SupportingClasses::State, 8                  # AuthMachine + Process
+      assert_number_of_objects AASM::SupportingClasses::Event, 12                 # AuthMachine + Process
+      assert_number_of_objects AASM::SupportingClasses::StateTransition, 20       # AuthMachine + Process
+      
+      Models.send(:remove_const, "Process") if Models.const_defined?("Process")
+      load File.expand_path(File.dirname(__FILE__) + '/../models/process.rb')
+      assert_equal 2, AASM::StateMachine.instance_variable_get("@machines").size  # AuthMachine + Process
+      assert_number_of_objects AASM::SupportingClasses::State, 8                  # AuthMachine + Process
+      assert_number_of_objects AASM::SupportingClasses::Event, 12                 # AuthMachine + Process
+      assert_number_of_objects AASM::SupportingClasses::StateTransition, 20       # AuthMachine + Process
+    end
+    
+  end
+  
+  private
+  
+    def assert_number_of_objects(clazz, num)
+      count = ObjectSpace.each_object(clazz) {}
+      assert_equal num, count, "#{num} expected, but we had #{count} #{clazz}"
+    end
+  
+end