From 5a8454221f0352aba37115c491220e016ea78872 Mon Sep 17 00:00:00 2001
From: Luke Roberts <luke@sqwiggle.com>
Date: Sun, 1 Mar 2015 20:08:45 -0800
Subject: [PATCH] reference state machine object and event in invalid
 transition error so end user can intelligently handle

---
 lib/aasm/aasm.rb             |  2 +-
 lib/aasm/errors.rb           | 13 ++++++++++++-
 spec/unit/transition_spec.rb |  7 ++++++-
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/lib/aasm/aasm.rb b/lib/aasm/aasm.rb
index 942f5dd..59d26fa 100644
--- a/lib/aasm/aasm.rb
+++ b/lib/aasm/aasm.rb
@@ -129,7 +129,7 @@ private
     end
 
     if AASM::StateMachine[self.class].config.whiny_transitions
-      raise AASM::InvalidTransition, "Event '#{event_name}' cannot transition from '#{aasm.current_state}'"
+      raise AASM::InvalidTransition.new(self, event_name) 
     else
       false
     end
diff --git a/lib/aasm/errors.rb b/lib/aasm/errors.rb
index 9b975d0..7e36fd8 100644
--- a/lib/aasm/errors.rb
+++ b/lib/aasm/errors.rb
@@ -1,5 +1,16 @@
 module AASM
-  class InvalidTransition < RuntimeError; end
+
+  class InvalidTransition < RuntimeError
+    attr_reader :object, :event_name
+    def initialize(object, event_name)
+      @object, @event_name = object, event_name
+    end
+
+    def message
+      "Event '#{event_name}' cannot transition from '#{object.aasm.current_state}'"
+    end
+  end
+
   class UndefinedState < RuntimeError; end
   class NoDirectAssignmentError < RuntimeError; end
 end
diff --git a/spec/unit/transition_spec.rb b/spec/unit/transition_spec.rb
index 12e70de..6d9194c 100644
--- a/spec/unit/transition_spec.rb
+++ b/spec/unit/transition_spec.rb
@@ -4,7 +4,12 @@ describe 'transitions' do
 
   it 'should raise an exception when whiny' do
     process = ProcessWithNewDsl.new
-    expect { process.stop! }.to raise_error(AASM::InvalidTransition)
+    expect { process.stop! }.to raise_error do |err|
+      expect(err.class).to eql(AASM::InvalidTransition)
+      expect(err.message).to eql("Event 'stop' cannot transition from 'sleeping'")
+      expect(err.object).to eql(process)
+      expect(err.event_name).to eql(:stop)
+    end
     expect(process).to be_sleeping
   end