notification should be string when enqueue

lib/devise/async/model.rb

@@ -2,6 +2,8 @@ module Devise
   module Models
     module Async
       extend ActiveSupport::Concern
+
+      STRINGIFYABLE_TYPES = [Symbol]
 
       included do
         # Register hook to send all devise pending notifications.
@@ -40,7 +42,8 @@ def send_devise_notification(notification, *args)
         # If the record isn't dirty (aka has already been saved) enqueue right away
         # because the callback has already been triggered.
         else
-          Devise::Async::Worker.enqueue(notification, self.class.name, self.id.to_s, *args)
+          args = stringify_args_values(args)
+          Devise::Async::Worker.enqueue(notification.to_s, self.class.name, self.id.to_s, *args)
         end
       end
 
@@ -49,7 +52,8 @@ def send_devise_pending_notifications
         devise_pending_notifications.each do |notification, args|
           # Use `id.to_s` to avoid problems with mongoid 2.4.X ids being serialized
           # wrong with YAJL.
-          Devise::Async::Worker.enqueue(notification, self.class.name, self.id.to_s, *args)
+          args = stringify_args_values(args)
+          Devise::Async::Worker.enqueue(notification.to_s, self.class.name, self.id.to_s, *args)
         end
         @devise_pending_notifications = []
       end
@@ -73,7 +77,22 @@ def add_current_locale_to_args(args)
         opts['locale'] = I18n.locale
         args.push(opts)
       end
+
+      def stringify_args_values(args)
+        args.each_with_object([]) do |a, acc|
+          arg = is_arg_hash?(a) ? a.deep_merge(a) {|_,_,v| is_val_stringifyable?(v) ? v.to_s : v} : a
+          acc << arg
+        end
+      end
 
+      def is_arg_hash?(arg)
+        arg.present? && arg.is_a?(Hash)
+      end
+
+      def is_val_stringifyable?(val)
+        # val is stringifyable if is one of the STRINGIFYABLE_TYPES
+        val.present? && STRINGIFYABLE_TYPES.include?(val.class)
+      end
     end
   end
 end