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Updating the Django Model and MQTT Daemon

We will update our "backend":

  1. Add the association field to the Lampi model, plus some helper methods and functions
  2. Modfiy the logic of the mqtt-daemon to publish the unassociated message

Update Django Model

We need to modify our Lampi database model to hold the association code.

Add a function to lampi/models.py to generate the association code (fill in the missing code):

def generate_association_code():
    # put your code here

Modify the Lampi model in lampi/models.py to include a new field association_code that is a Django CharField with max_length=32 and default=generate_association_code. The default with a Python "callable" (a function in this case, that you defined above) will cause generate_association_code() to be called whenever a new instance of Lampi is created, so each instance will get a unique, 32 character code.

Add a helper method to the Lampi model (there's a convention in Python that method names for "private" methods are prefixed with an underscore _):

    def _generate_device_association_topic(self):
        return 'devices/{}/lamp/associated'.format(self.device_id)

This will return the MQTT Topic for the particular device the Lampi instance represents.

Add two additional methods to the Lampi model, and fill in the missing code:

    def publish_unassociated_msg(self):
        # send association MQTT message
        assoc_msg = {}
        # your code goes here

    def associate_and_publish_associated_msg(self,  user):
        # update Lampi instance with new user
        # publish associated message
        assoc_msg = {}
        # your code goes here

For simplicity, when publishing MQTT messages from your Lampi class in these two methods, use the Paho Publish Single module function (this will take care of creating a client, connecting to the broker, and publishing your message, all in one call). Be sure to provide all of the parameters required (hostname, port number, etc.).

Test your new Lampi class in the Django shell. You can make up device_id values for testing (just make sure they are 12 characters long).

NOTE: do not forget to create the new database migrations, and apply those migrations to your database!

Update MQTT Daemon

Modify Web/lampisite/lampi/management/commands/mqtt-daemon.py to invoke the new publish_unassociated_msg() method on the Lampi object after it is created and saved.

Remember, your mqtt-daemon is being manged by supervisord, so you might want to stop it, using supervisorctl and run it manually for testing and debug.

Testing the New Backend Functionality

You can test this with your LAMPI, and by creating test devices (to fool the mqtt-daemon into creating a new Lampi instance and publishing a message, publish a message with mosquitto_pub to $SYS/broker/connection/<SOME MADE UP DEVICE_ID>_broker/state with a value of 1).

From time to time, you may need to remove Retained messages from your brokers (EC2 or LAMPI) - you can use the -r and -n command-line options with mosquitto_pub.

You can delete a Lampi object from the database like so:

$ ./manage.py shell
Python 3.5.2 (default, Nov 23 2017, 16:37:01)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> from lampi.models import Lampi
>>> Lampi.objects.all()
<QuerySet [<Lampi: b827ebba0387: My LAMPI>]>
>>> d = Lampi.objects.get(device_id='b827ebba0387')
>>> d
<Lampi: b827ebba0387: My LAMPI>
>>> d.delete()
(1, {'lampi.Lampi': 1})
>>> Lampi.objects.all()
<QuerySet []>

NOTE: Remember that there are a few things going on here (and some are "stateful" that is, they have a peristent record in the database and/or the MQTT broker(s) as retained messages):

  • the mqtt-daemon is an "edge" detector (it detects new devices being bridged to the Mosquitto Broker by monitoring the $SYS/broker/connection/+/state subscription with payloads of b'1' (byte arrays with an ASCII value of numeral 1); only new, unassociated devices will cause the unassociated message to be published
  • when the mqtt-daemon detects a new (previously unknown) LAMPI device, it creates a new Lampi database model instance, and publishes an unassociated mesage as a retained message
  • when the LAMPI device receives an unassociated message, it displays the popup

Testing this set of behaviors will likely require you to periodically remove Lampi objects from the database, monitor MQTT messages, publish MQTT messages, and periodically remove retained messages from the broker(s).

You should be able to manually associate a device with a user using the Django shell and fully test the LAMPI and "backend" systems behavior at this point. All that is remaining is to build a web UI in the next section.

Next up: go to Updating Django

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