Counting occurences of subclasses in Django

Counting occurences of subclasses in Django

As a Django learning project I have began CONTMAN, a simple mobile content distribution system, and as expected I have ran into many different kinds of problems that I have thought it would be interesting to keep a log of... and this is it.

This might be the classic case of "When all you have is a hammer everything looks like a nail" as when I was thinking on how to design the project I came across Django inheritance and polymorphism features. "Aha! this is EXACTLY what I need" I said to myself. Now I am not so sure, but I am perhaps one of the most stubborn people in the world, so I kept going in that direction.

So I thought. I have two, maybe three different kinds of content I want to be able to distribute with this platform, for now I have "Ringtones" and "Wallpapers" (NB: I know it is not bleeding edge stuff, but keep in mind this was for learning, and I could not think of anything else at the time), and both of them are Content.

So we have a parent Content Class

class Content(models.Model):
    name = models.CharField(max_length=100)
    created_on = models.DateTimeField('created', auto_now_add = True )
    keyword = models.CharField(max_length=100)

And to children subclasses

class Ringtone(Content):
    artist = models.ManyToManyField(Artist)
    data_file = models.FileField(upload_to="uploads")

class Wallpaper(Content):
    category = models.ForeignKey(Category)
    data_file = models.ImageField(upload_to="uploads")

Ok, so there is a whole bunch of other stuff there but this is enough to get the point of this post across.

To me this was going to be very helpful in a lot of ways to mention some:

• Delivering content would be as easy as pointing to Content.data_file (which you can't is it is clearly stated at the very end of this doc entry.)
• The admin app would let me easily create a manager for uploading the file depending on the type (see comment above)
• Creating reports would be very easy, just create a query set from the Content model.

This last point is the one we will be discussing. 

So it turns out that

a = Content.objects.get(pk=1)

returns a Content object, so you cannot count or fetch content using  "a.data_file".  So how can I do a simple report showing the count of each type of content available? The easy (lame) way could be to simple do two queries of Ringtone and Wallpapers as we only have those and leave Content alone. But this is not nice, in fact it reeks of bad code and self-pity. What if I later add support for "applications" and "games" Content types?

So after a nice session of Django docs + stack overflow + google I think I came across a very simple but effective solution here in stack overflow

The idea is to take advantage of the Contenttypes framework to keep track of what kind of model each object represents. This way we can aggregate easily on our query a create this type of report. So we modify the base (parent) class like this:

class Content(models.Model):
    name = models.CharField(max_length=100)
    created_on = models.DateTimeField('created', auto_now_add = True )
    keyword = models.CharField(max_length=100)
    content_type = models.ForeignKey("contenttypes.ContentType")

With this then we can easily do this:

class Content(models.Model):
>>>Content.objects.values('content_type__name').annotate(hits=Count("content_type"))
[{'hits': 1, 'content_type__name': u'ringtone'}, {'hits': 2, 'content_type__name': u'wallpaper'}]