>> class Tracker(models.Model):
>> notifications = models.ForeignKey(Notification)
>>
>> class Notification(models.Model):
>> # Common fields
>> pass
>>
>> class EmailNotification(Notification):
>> pass
>>
>> class SmsNotification(Notification):
>> pass
>>
>>
>> # I would be able to iterate over all types of notification
>> for n in tracker.notifications:
>> print n
>
> Not trivially, purely in SQL, at the moment. You really need to union
> two sets of results together, since they're querying different tables.
> Lots of history here, but basically, it's a hard problem to solve
> efficiently. One day we might do it.
If one is willing to be roped to PostgreSQL, it does support such
a scheme with table-inheritance. You'd have to hack the table
definitions by hand instead of letting "manage.py syncdb" do it
for you. Additionally, Django's ORM knows nothing about this DB
hack, so virtual methods won't work like one might hope:
class Notification(models.Model):
def __unicode__(self):
return u"Generic notification:" % self.common_field
class EmailNotification(Notification):
def __unicode__(self):
return u"Email: %s" % self.some_email_field
class SMSNotification(Notification):
def __unicode__(self):
return u"SMS: %s" % self.some_sms_field
for n in tracker.notifications:
print unicode(s)
You'll end up with every item being treated as a Notification
object (and thus printing "Generic notification" for all of
them), rather than getting the notification-specific methods.
> On the other hand, since querysets are iterators, you can construct two
> querysets and then use itertools.chain to combine them and have it
> appears as a single iterator.
A slightly less DRY solution than ideal (ideal=solving the VERY
complex STI problem in Django's ORM), but more portable across
database back-ends, less fragile, and available out of the box.
Another alternative might be using Django's built-in generic
relations.
-tim
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