RapidSMS Tutorial Part 3

So far, nothing we’ve done really requires Django. Let’s create a RapidSMS application that uses Django’s abilities to store data.

We’ll create an extremely simple voting application. It will understand two messages: VOTE <choice> will add a vote for the specified choice, and RESULTS will respond with the current number of votes for each choice.

(Please notice that this application is more appropriate for a group to choose a place to go to lunch than for anything serious. It makes no attempt whatsoever to enforce any of the controls a real election would need.)

Create the application

Create a new Django application. Let’s call it “voting”:

$ python manage.py startapp voting

The models

This application is so simple that we’ll only need one model. We’ll call it Choice, and there will be one instance for each possible choice. Each record will contain the name of the choice and the number of votes for it so far.

# voting/models.py
from django.db import models
class Choice(models.Model):
    name = models.CharField(max_length=40, unique=True)
    votes = models.IntegerField(default=0)

Application Design

Even a simple application like this can demonstrate an important design principle for RapidSMS applications.

Instead of adding to a candidate’s vote count each time a vote arrived, we could instead have created a Vote model and stored a record of each vote. That seems like a little simpler way to handle an incoming vote.

However, if we did that, whenever we needed the results we would have to query every record in our database to count up the votes for each choice. There are SQL queries that can simplify doing that, but the database still has to look at every record. And the next time we wanted the results, we’d have to do that again.

We’re better off doing a little more processing on each message, if that can save us a lot of work later on.

Admin

We’ll want to use the Django admin to set up our choices. Typically you would need to make a few simple changes to your project’s urls.py, but the RapidSMS project template has already done that for us. So all we need to do is add admin.py to our application:

# voting/admin.py
from django.contrib import admin
from .models import Choice
admin.site.register(Choice)

The results handler

Let’s start with the simpler message to handle, RESULTS. This is easily implemented as a RapidSMS keyword handler. Let’s create a file handlers.py to contain our handlers, and write a handler that responds with the current votes.

# voting/handlers.py
from rapidsms.contrib.handlers import KeywordHandler
from .models import Choice
class ResultsHandler(KeywordHandler):
    keyword = "results"
    def help(self):
        """help() gets invoked when we get the ``results`` message
        with no arguments"""
        # Build the response message, one part per choice
        parts = []
        for choice in Choice.objects.all():
            part = "%s: %d" % (choice.name, choice.votes)
            parts.append(part)
        # Combine the parts into the response, with a semicolon after each
        msg = "; ".join(parts)
        # Respond
        self.respond(msg)
    def handle(self, text):
        """This gets called if any arguments are given along with
        ``RESULTS``, but we don't care; just call help() as if they
        passed no arguments"""
        self.help()

If the choices are “Moe”, “Larry”, and “Curly”, the response to a RESULTS message might look like Moe: 27; Larry: 15; Curly: 98.

The vote handler

The VOTE message is slightly more work. If we receive VOTE xxxx where xxx is one of the choices (case-insensitive), we want to increment the votes for choice xxx and respond telling the user that their vote has been counted. If we receive any other message starting with VOTE, we’ll respond with some help to tell them how the command works and what the choices are.

# voting/handlers.py (continued)
from django.db.models import F
class VoteHandler(KeywordHandler):
    keyword = "vote"
    def help(self):
        """Respond with the valid commands.  Example response:
        ``Valid commands: VOTE <Moe|Larry|Curly>``
        """
        choices = "|".join(Choice.objects.values_list('name', flat=True))
        self.respond("Valid commands: VOTE <%s>" % choices)
    def handle(self, text):
        text = text.strip()
        # look for a choice that matches the attempted vote
        try:
            choice = Choice.objects.get(name__iexact=text)
        except Choice.DoesNotExist:
            # Send help
            self.help()
        else:
            # Count the vote. Use update to do it in a single query
            # to avoid race conditions.
            Choice.objects.filter(name__iexact=text).update(votes=F('votes')+1)
            self.respond("Your vote for %s has been counted" % text)

Settings

We need to add our Django app to INSTALLED_APPS and our handlers to RAPIDSMS_HANDLERS:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
INSTALLED_APPS = (
   [...]
    # RapidSMS
    "voting",
   [...]
    "rapidsms.contrib.default",  # Must be last
)
RAPIDSMS_HANDLERS = [
    [...]
    "voting.handlers.ResultsHandler",
    "voting.handlers.VoteHandler",
    [...]
]

Update database

We’ve added a new model, so we need to update our database to include it:

$ python manage.py syncdb
Syncing...
Creating tables ...
Creating table voting_choice
[... rest of output omitted ...]

Create some choices

Now it’s time to start our application and create some choices to vote for.

$ python manage.py runserver
Validating models...
0 errors found
May 07, 2013 - 08:28:44
Django version 1.5.1, using settings 'rapidsms_tut.settings'
Development server is running at http://127.0.0.1:8000/
Quit the server with CONTROL-C.

Go to http://127.0.0.1:8000/admin/voting/choice/, login as the superuser you created in part 1 of the tutorial, and you should be able to add some choices.

Vote

Let’s start by checking that there are no votes. Go to the message tester application (http://127.0.0.1:8000/httptester/) and send the message RESULTS. You should see a response showing no votes, something like this:

05/07/2013 8:30 a.m.        349911« Moe: 0; Larry: 0; Curly: 0
05/07/2013 8:30 a.m.        349911» RESULTS

(Recall that the messages are shown in reverse order.)

Now let’s cast a vote. Send VOTE Moe and you should see something like:

05/07/2013 8:32 a.m.        349911« Your vote for Moe has been counted
05/07/2013 8:32 a.m.        349911» VOTE Moe

and if you check the results again:

05/07/2013 8:33 a.m.        349911« Moe: 1; Larry: 0; Curly: 0
05/07/2013 8:33 a.m.        349911» RESULTS

Continue with RapidSMS Tutorial Part 4.

Previous topic

Frequently Asked Questions

This Page