var userSchema = new mongoose.Schema({
 firstname: String,
 surname: String,
 email: String,
});

var appointmentSchema = new mongoose.Schema({
 userID: String,
 type: Number,
 time: Date
});

Appointment.aggregate([
        {
            $match: {"type": 1}
        },
        {
            $lookup:{
                from: "users",
                localField: "userID",
                foreignField: "_id",
                as: "appointmentUser"
            }
        }], function(err,foundAppointmentUser) {
                if(err){
                    console.log(err);
                } else {
                    console.log(foundAppointmentUser);
                }
        });

{
    "_id": {
        "$oid": "5bcc8eac7ac5980bfa365183"
    },
    "userID": "5bb4d1945480e60771ccde5a",
    "type": 1,
    "time": {
        "$date": "2018-10-22T10:00:00.000Z"
    },
    "__v": 0
}

{
    "_id": {
        "$oid": "5bb4d1945480e60771ccde5a"
    },
    "firstname": "Stu",
    "surname": "Test 999",
    "email": "stu@stu.com",
    "created": {
        "$date": "2018-10-03T14:26:28.815Z"
    },
    "__v": 0
}

[ { _id: 5bcc8eac7ac5980bfa365183,
    userID: '5bb4d1945480e60771ccde5a',
    type: 1,
    time: 2018-10-22T10:00:00.000Z,
    __v: 0,
    appointmentUser: [] } ]