Polymorphism in AppEngine Datastore Models

There is a problem with inherited classes in AppEngine

Let's suppose we have the following models:

class Master(db.Model):
  mp = db.StringProperty()

class Detail(db.Model):
  dp = db.StringProperty()
  master = db.ReferenceProperty(Master)

When these are declared, Datastore appends automatically Detail_set property to the Master. So if we made

m=Master(mp='foo')
m.put()
d1=Detail(dp='bar', master=m)
d1.put()
d2=Detail(dp='zee', master=m)
d2.put()

then we have m.Detail_set property which will fetch [d1, d2]. But if we define

class MoreDetail (Detail):
  mdp=db.StringProperty()

d3=MoreDetail (dp='org', mdp='jee', master=m)
d3.put()

then m.detail_set will fetch the third d3 but de-serialize it as Detail instead of MoreDetail class. Here is how I checked it:

>>> for d in m.detail_set.fetch(10):
...  print d.properties()
{'master': <ReferenceProperty object at 0x018B8330>, 'dp': <StringProperty object at 0x023A8C10>}
{'master': <ReferenceProperty object at 0x018B8330>, 'dp': <StringProperty object at 0x023A8C10>}
{'master': <ReferenceProperty object at 0x018B8330>, 'dp': <StringProperty object at 0x023A8C10>}

One of these objects should have an mdp property defined in MoreDetail, but that did not happen.