Updating Object State in MongoDB via MQTT & Node.js - node.js

Question

I am building a node.js API to manage IOT devices. The IOT device have a state which can be true or false, depending on user input. The server subscribes the user to the device topic, once the user adds the device to their account. I am able to toggle the state easily, with the following code :
exports.toggleState = async (req, res, next) => {
let deviceID = req.params.id
let userId = req.userId
try {
let userdata = await User.findById(userId)
let deviceList = userdata.deviceList
deviceList = deviceList.map(el => {
return el.toString()
})
if (deviceList.indexOf(deviceID) !== -1) {
Device.findById(mongoose.Types.ObjectId(deviceID)).then(result => {
result.state = !result.state
return result.save()
})
.then(result => {
req.mqttClient.publish(`${result.topic}`, `${result.state}`, { qos: 0, retain: true }, (error) => {
if (error) {
throw error
}
})
res.status(201).json({ message: 'Device State Changed', data: result })
})
.catch(err => {
console.log(err)
throw err
})
}
else {
res.status(500).json({ message: 'Device not linked to your account!' })
}
} catch (err) {
if (!err.statusCode) {
err.statusCode = 500;
}
next(err);
}
};
I have added mqtt to make it easier for device side operation (Arduino based).
The challenge I am facing is that if the state changes from the device side (via hardware input), how do I keep the device state updated on the database through the Mqtt server? Some thing like :
Device.findById(deviceID).then(result => {
req.mqttClient.on('message', (`${result.topic}`, payload) => {
console.log('Received Message:', `${result.topic}` , payload.toString())
})
result.state = payload.toString()
return result.save()
}.then(()=>{
console.log('State successfully updated')
}
Problem is I don't know where do I plug this code so that it runs continuously, monitoring incoming messages and updating device state. Would greatly appreciate any help in this.