Come popolare in questo caso Mongoose?

Prova:

  notifications: {
    notifiId: {type: Schema.Types.ObjectId, ref: 'Notifications' },
    viewed: { type: Number, default: 0 }
  },

e per trovarlo:

model_Variable.findById(req.params.id).populate("notifiId").exec(function (err, 
callBack) {});