Question

Basically I have a Base class called “Program”. I then have more specific program model types that use Program as a base class. For 99% of my needs, I don’t care whether or not a Program is one of the specific child types. Of course there’s that 1% of the time that I do want to know if it’s one of the children.

The problem is that if I have let’s say, a SwimProgram model and a CampProgram model using Program as their base, that it’s problematic to find out what they are without a bunch of try/except blocks. What I want is something like the following:

program = models.Program.objects.get(id=15)
if program.swimprogram:
    ## do stuff
elif program.campprogram:
    ## do stuff
else:
    ## do other stuff

Of course this throws DoesNotExist exceptions. I could either use try/excepts which are uglier, or I could have Program have a ‘type’ field that the children set on save. Both are doable, but I’m curious if anyone has any better methods.

Answer 1

Have you tried hasattr()? Something like this:

if hasattr(program, 'swimprogram'):
    # ...
elif hasattr(program, 'campprogram'):
    # ...

If you are unsure about this approach, try it out in a simple test app first. Here are two simple models that should show if it will work for you and the version of django that you are using (tested in django-1.1.1).

class Archive(models.Model):
    pub_date = models.DateField()

    def __unicode__(self):
        return "Archive: %s" % self.pub_date

class ArchiveB(Archive):
    def __unicode__(self):
        return "ArchiveB: %s" % self.pub_date

And then giving it a spin in the shell:

> a_id = Archive.objects.create(pub_date="2010-10-10").id
> b_id = ArchiveB.objects.create(pub_date="2011-11-11").id
> a = Archive.objects.get(id=a_id)
> b = Archive.objects.get(id=b_id)
> (a, b) # they both look like archive objects
(<Archive: Archive: 2010-10-10>, <Archive: Archive: 2011-11-11>)
> hasattr(a, 'archiveb')
False
> hasattr(b, 'archiveb') # but only one has access to an ArchiveB
True