nodejs使用promises来处理多个回调

Question

我正在尝试了解如何异步或承诺或任何使我的代码更好的东西。这是我的代码的一个例子

Bus.findOne({_id: bus_id}, function (err, bus) {
if (err) {
    res.json({status: "error", message: "please enter a valid bus_id"});
    return;
}
bus.language = record.language ? record.language : bus.language;
bus.bus_number = record.bus_number  ? record.bus_number : bus.bus_number;
bus.bus_driver = bus_driver ? bus_driver : bus.bus_driver;
bus.bus_details = bus_details ? bus_details : bus.bus_details;
bus.trips = record.trips ? record.trips : bus.trips;
bus.location = record.location ? record.location : bus.location;
bus.save(function (err, bus) {
    if (err) {
        res.json({status: "error", error: errors, message: "bus wasn't saved"});
        return;
    } else {
        User.findOne({refid:bus_id},function(err,doc){
            if (err) {
                res.json({status: "error", error: "bus user update was not saved"});
                return;
            } else if(doc){
                doc.name =  "bus number"+ '' + bus.bus_number;
                doc.refid = bus.id;
                doc.save(function(err){
                    if (err) {
                        res.json({status: "error", error: "bus user update was not saved"});
                        return;
                    }else{
                        res.json({status: "success", message: "bus was successfully updated", bus: bus});
                        return;
                    }
                });
            }else{
                res.json({status:"error",message:"an error with finding user"});
                return;
            }
        });
    }
});
  });

这就是我最终的结果

Bus.findOne({_id:bus_id}).exec().then(function(bus){
    bus.language = record.language ? record.language : bus.language;
    bus.bus_number = record.bus_number  ? record.bus_number : bus.bus_number;
    bus.bus_driver = bus_driver ? bus_driver : bus.bus_driver;
    bus.bus_details = bus_details ? bus_details : bus.bus_details;
    bus.trips = record.trips ? record.trips : bus.trips;
    bus.location = record.location ? record.location : bus.location;
    return bus.save().exec();
}).then(function(bus){
    return User.findOne({refid:bus._id}).exec();
}).then(function(doc){
    doc.name =  "bus number"+ '' + bus.bus_number;
    doc.refid = bus.id;
    return User.save().exec();
}).then(function(){
    res.json({status:"success",message:"bus and associated user has been removed"});
}).catch(function(error){
    res.json({status:"error",message:"an error with finding user"});
});

但是服务器一直在捕捉错误，我知道我做错了什么但我不知道错误在哪里