denormalize_destroy_to_* trigger before destroy

The to_alize destroy callbacks used to be a after_destroy
callback which failed when relations specified a :nullify
dependency - since mongoid would nullify all relations before
the destroy and the after_destroy callback would find no children
after the destroy op to sync.

Changed the callback to be triggered before the destroy op, except
in the case of a :restrict dependency where a before_destroy callback
would pre-maturely sync the child objects even though the destroy op
may never happen

Author: pranav-g10

lib/mongoid/alize/to_callback.rb

@@ -132,7 +132,11 @@ def set_callback
 
       def set_destroy_callback
         unless callback_attached?("destroy", aliased_destroy_callback_name)
-          klass.set_callback(:destroy, :after, aliased_destroy_callback_name)
+          if self.metadata.dependent == :restrict
+            klass.set_callback(:destroy, :after, aliased_destroy_callback_name)
+          else
+            klass.set_callback(:destroy, :before, aliased_destroy_callback_name)
+          end
         end
       end