Combining Querysets from different, but similar models

I'm working on a site that has a requirement for a "recent activity"
feed on the homepage. Usually, I'd just do a MyFoo.objects.order_by('-
created') and all would be well. But "activity" is actually from
several models. They all have some fields in common, like the
timestamp, since they all have the same abstract base class, but
they're definitely different things.

What's the cheapest way to do my queries and mix the results into one
feed? Ideally, the name of the model, or some other differentiating
attribute would be exposed so that I can use it in my template loop as
a style hook. Models in question are below...

class TrackableModel(models.Model):
created = models.DateTimeField(auto_now_add=True)
modified = models.DateTimeField(auto_now=True)
created_by = models.ForeignKey(User, blank=True, null=True)
history = models.TextField(blank=True)
notes = models.TextField(blank=True)
is_active = models.BooleanField(blank=True, default=True)

class Meta:
abstract = True

class Review(TrackableModel):
book = models.ForeignKey('Textbook')
course = models.ForeignKey('TaughtCourse', blank=True, null=True)
should_buy = models.BooleanField(blank=True)
rating_prep = models.DecimalField(blank=True, null=True,
max_digits=2, decimal_places=1)
rating_hw = models.DecimalField(blank=True, null=True,
max_digits=2, decimal_places=1)
rating_inclass = models.DecimalField(blank=True, null=True,
max_digits=2, decimal_places=1)
can_share = models.BooleanField(blank=True, default=True)
comments = models.TextField(blank=True)

class MarketplacePosting(TrackableModel):
book = models.ForeignKey('Textbook', blank=True, null=True)
school = models.ForeignKey('School', blank=True, null=True)
location = models.CharField(max_length=255, blank=True,
verbose_name="Sale location")
condition = models.CharField(max_length=15, blank=True,
choices=CONDITION_CHOICES)
contains_highlighting = models.BooleanField(blank=True)
price = models.DecimalField(blank=True, null=True, max_digits=6,
decimal_places=2)
comments = models.TextField(blank=True)
is_sold = models.BooleanField(blank=True)

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