added array to tag type

message.model.js

 //ObjectId = Schema.ObjectId;
 
 const MessageSchema = mongoose.Schema({
-    //_id: ObjectId,
-    /*id: {
-        type: String,
-        getter: function(val) { return this._id.toString(); },
-        unique: true
-    },*/
-    //id: {type: String, required: true},
     subject: { type: String, required: true },
     message: { type: String, required: true },
     user: { type: String, required: true },
-    tag: {type: String },
+    tag: [{type: String, index: true }],
+    //createtime: { type: Date, default: Date.now },
 });
 
 module.exports = mongoose.model('Message', MessageSchema);