Handle :on => [] correctly.

lib/paper_trail/has_paper_trail.rb

@@ -70,14 +70,14 @@ module PaperTrail
             :order      => self.paper_trail_version_class.timestamp_sort_order
         end
 
-        options_on = Array(options[:on]) # so that a single symbol can be passed in without wrapping it in an `Array`
-        after_create  :record_create, :if => :save_version? if options_on.empty? || options_on.include?(:create)
-        if options_on.empty? || options_on.include?(:update)
+        options_on = Array(options[:on]) unless options[:on].nil? # so that a single symbol can be passed in without wrapping it in an `Array`
+        after_create  :record_create, :if => :save_version? if options_on.nil? || options_on.include?(:create)
+        if options_on.nil? || options_on.include?(:update)
           before_save   :reset_timestamp_attrs_for_update_if_needed!, :on => :update
           after_update  :record_update, :if => :save_version?
           after_update  :clear_version_instance!
         end
-        after_destroy :record_destroy, :if => :save_version? if options_on.empty? || options_on.include?(:destroy)
+        after_destroy :record_destroy, :if => :save_version? if options_on.nil? || options_on.include?(:destroy)
 
         # Reset the transaction id when the transaction is closed
         after_commit :reset_transaction_id

test/unit/model_test.rb

@@ -1203,6 +1203,22 @@ class HasPaperTrailModelTest < ActiveSupport::TestCase
         assert_equal 'destroy', @fluxor.versions.last.event
       end
     end
+    context 'on []' do
+      setup do
+        Fluxor.reset_callbacks :create
+        Fluxor.reset_callbacks :update
+        Fluxor.reset_callbacks :destroy
+        Fluxor.instance_eval <<-END
+          has_paper_trail :on => []
+        END
+        @fluxor = Fluxor.create
+        @fluxor.update_attributes :name => 'blah'
+        @fluxor.destroy
+      end
+      should 'not have any versions' do
+        assert_equal 0, @fluxor.versions.length
+      end
+    end
     context 'allows a symbol to be passed' do
       setup do
         Fluxor.reset_callbacks :create