Model Inheritance and ModelForms

I've been scouring Google and the Django documentation and I can't
figure out how to do this. I'm working on an inventory management
system for my shop. My inventory models.py similar to the following:

class ItemType( models.Model ):
def __unicode__( self ):
return "blah blah blah"

itemType = models.CharField( max_length = 32 )
isBook = models.BooleanField()

class InventoryItem( models.Model ):
def __unicode__( self ):
return "blah"

itemType = models.ForeignKey( ItemType )
description = models.CharField( max_length = 256 )

class InventoryBook( InventoryItem ):
def __unicode__( self ):
return "blah blah"

title = models.CharField( max_length = 64 )

In my web app, I create a ModelForm based on InventoryItem and present
that to the user. Upon submission, the POST data is used to create an
instance of InventoryItem. I then check to see if
inventoryItem.itemType.isBook() is True - if True, I want to cast
inventoryItem to an InventoryBook type so I can set the extra fields
and call save() so that it creates records in both tables in the MySQL
database.

I started trying to add a method to InventoryItem that would return an
InventoryBook instance after being given the title string, but that
doesn't work because Python doesn't have prototyping/forward
declarations.

Am I approaching this completely wrong, or am I just overlooking
something simple? Any advice or links to relevant documentation would
be *much* appreciated. Thanks!

-Alex

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