Mongoose: use $type as the typeKey (#10789)

* use $type as the typeKey in mongoose

* fix and add tests

website/server/models/userNotification.js

@@ -37,7 +37,7 @@ const Schema = mongoose.Schema;
 
 export let schema = new Schema({
   id: {
-    type: String,
+    $type: String,
     default: uuid,
     validate: [v => validator.isUUID(v), 'Invalid uuid.'],
     // @TODO: Add these back once we figure out the issue with notifications
@@ -45,25 +45,26 @@ export let schema = new Schema({
     // required: true,
   },
   type: {
-    type: String,
+    $type: String,
     // @TODO: Add these back once we figure out the issue with notifications
     // See Fix for https://github.com/HabitRPG/habitica/issues/9923
     // required: true,
     enum: NOTIFICATION_TYPES,
   },
-  data: {type: Schema.Types.Mixed, default: () => {
+  data: {$type: Schema.Types.Mixed, default: () => {
     return {};
   }},
   // A field to mark the notification as seen without deleting it, optional use
   seen: {
-    type: Boolean,
+    $type: Boolean,
     // required: true,
     default: () => false,
   },
 }, {
   strict: true,
   minimize: false, // So empty objects are returned
-  _id: false, // use id instead of _id
+  _id: false, // use id instead of _id,
+  typeKey: '$type', // So that we can use fields named `type`
 });
 
 /**