handle edge cases when a state machine is accessed which doesn't exist

PLANNED_CHANGES.md

@@ -11,8 +11,6 @@
 # Currently working on
 
  * add support for multiple state machines per class
-   * what happen's if someone accesses `aasm`, but has defined a
-     state machine for `aasm(:my_name)`?
 
 
 # Changes so far
@@ -29,4 +27,6 @@
      * _Mongoid_
      * _MongoMapper_
      * _Sequel_
+   * what happen's if someone accesses `aasm`, but has defined a
+     state machine for `aasm(:my_name)`?
  * drop support for find_in_state, count_in_state, calculate_in_state, with_state_scope

lib/aasm/aasm.rb

@@ -47,6 +47,9 @@ module AASM
 
   # this is the entry point for all instance-level access to AASM
   def aasm(name=:default)
+    unless AASM::StateMachine[self.class][name.to_sym]
+      raise AASM::UnknownStateMachineError.new("There is no state machine with the name '#{name}' defined in #{self.class.name}!")
+    end
     @aasm ||= {}
     @aasm[name.to_sym] ||= AASM::InstanceBase.new(self, name.to_sym)
   end

lib/aasm/errors.rb

@@ -1,5 +1,7 @@
 module AASM
 
+  class UnknownStateMachineError < RuntimeError; end
+
   class InvalidTransition < RuntimeError
     attr_reader :object, :event_name, :state_machine_name
 

spec/unit/edge_cases_spec.rb

@@ -0,0 +1,16 @@
+require 'spec_helper'
+
+describe "edge cases" do
+  describe "for classes with multiple state machines" do
+    it "allows accessing a multiple state machine class without state machine name" do
+      # it's like starting to define a new state machine within the
+      # requested class
+      expect(SimpleMultipleExample.aasm.states.map(&:name)).to be_empty
+    end
+
+    it "do not know yet" do
+      example = ComplexExampleMultiple.new
+      expect { example.aasm.states.inspect }.to raise_error(AASM::UnknownStateMachineError)
+    end
+  end
+end