Merge pull request #505 from gocardless/504-last_transition-cache-doe…

…snt-work-for-newly-created-objects

Make sure `nil` is accepted as cached value

lib/statesman/adapters/active_record.rb

@@ -52,7 +52,7 @@ def create(from, to, metadata = {})
 
         raise
       ensure
-        @last_transition = nil
+        remove_instance_variable(:@last_transition)
       end
 
       def history(force_reload: false)
@@ -65,18 +65,18 @@ def history(force_reload: false)
         end
       end
 
-      # rubocop:disable Naming/MemoizedInstanceVariableName
       def last(force_reload: false)
         if force_reload
           @last_transition = history(force_reload: true).last
+        elsif instance_variable_defined?(:@last_transition)
+          @last_transition
         else
-          @last_transition ||= history.last
+          @last_transition = history.last
         end
       end
-      # rubocop:enable Naming/MemoizedInstanceVariableName
 
       def reset
-        @last_transition = nil
+        remove_instance_variable(:@last_transition)
       end
 
       private

spec/statesman/adapters/active_record_spec.rb

@@ -308,9 +308,9 @@
       described_class.new(MyActiveRecordModelTransition, model, observer)
     end
 
-    before { adapter.create(:x, :y) }
-
     context "with a previously looked up transition" do
+      before { adapter.create(:x, :y) }
+
       before { adapter.last }
 
       it "caches the transition" do
@@ -376,6 +376,16 @@
         expect(adapter.last.to_state).to eq("y")
       end
     end
+
+    context "without previous transitions" do
+      it "does query the database only once" do
+        expect(model.my_active_record_model_transitions).
+          to receive(:order).once.and_call_original
+
+        expect(adapter.last).to eq(nil)
+        expect(adapter.last).to eq(nil)
+      end
+    end
   end
 
   it "resets last with #reload" do