Update Comment.js, line 7

models/Comment.js

@@ -4,7 +4,7 @@ var CommentSchema = new mongoose.Schema({
   body: String,
   author: { type: mongoose.Schema.Types.ObjectId, ref: 'User' },
   article: { type: mongoose.Schema.Types.ObjectId, ref: 'Article' }
-}, {timestamps: true});
+}, {timestamps: true, usePushEach: true});
 
 // Requires population of author
 CommentSchema.methods.toJSONFor = function(user){