MONGOID-5833 Fix Document#touch (#6079)

* Changing how we determine if a field should have a write on touch

* add test to ensure code path is not reached

* add a comment for test

Author: kyleburgess2025

lib/mongoid/touchable.rb

@@ -39,7 +39,7 @@ def touch(field = nil)
           # seems to not always/ever be set here. See MONGOID-5014.
           _parent.touch
 
-          if field
+          if field.present?
             # If we are told to also touch a field, perform a separate write
             # for that field. See MONGOID-5136.
             # In theory we should combine the writes, which would require

spec/mongoid/touchable_spec.rb

@@ -149,6 +149,18 @@
             building.updated_at.should == update_time
           end
         end
+
+        context 'when not updating an additional field' do
+          it 'does not call set_field_atomic_updates' do
+            # Regression test for MONGOID-5833
+            entrance
+            update_time
+            expect(entrance).not_to receive(:set_field_atomic_updates)
+            entrance.touch
+
+            entrance.updated_at.should == update_time
+          end
+        end
       end
 
       context "when the document is referenced" do