Mongoose 4.x (#9928)

* update mongoose to ^4.x

* another fix

website/server/models/task.js

@@ -54,6 +54,23 @@ export let TaskSchema = new Schema({
         return !validator.isUUID(val);
       },
       msg: 'Task short names cannot be uuids.',
+    }, {
+      validator (alias) {
+        return new Promise((resolve, reject) => {
+          Task.findOne({ // eslint-disable-line no-use-before-define
+            _id: { $ne: this._id },
+            userId: this.userId,
+            alias,
+          }).exec().then((task) => {
+            let aliasAvailable = !task;
+
+            return aliasAvailable ? resolve() : reject();
+          }).catch(() => {
+            reject();
+          });
+        });
+      },
+      msg: 'Task alias already used on another task.',
     }],
   },
   tags: [{
@@ -193,20 +210,6 @@ TaskSchema.methods.scoreChallengeTask = async function scoreChallengeTask (delta
 
 export let Task = mongoose.model('Task', TaskSchema);
 
-Task.schema.path('alias').validate(function valiateAliasNotTaken (alias, respond) {
-  Task.findOne({
-    _id: { $ne: this._id },
-    userId: this.userId,
-    alias,
-  }).exec().then((task) => {
-    let aliasAvailable = !task;
-
-    respond(aliasAvailable);
-  }).catch(() => {
-    respond(false);
-  });
-}, 'Task alias already used on another task.');
-
 // habits and dailies shared fields
 let habitDailySchema = () => {
   return {history: Array}; // [{date:Date, value:Number}], // this causes major performance problems