Django Model Inheritance And Foreign Keys

One way to do this is to add an intermediate class as follows:

class A(Model):
    class Meta(Model.Meta):
        abstract = True
    # common definitions here

class Target(A):
    # this is the target for links from D - you then need to access the 
    # subclass through ".b" or ".c"
    # (no fields here)

class B(Target):
    # additional fields here

class C(Target):
    # additional fields here        

class D(A):
    b_or_c = ForeignKey(Target)
    def resolve_target(self):
        # this does the work for you in testing for whether it is linked 
        # to a b or c instance
        try:
            return self.b_or_c.b
        except B.DoesNotExist:
            return self.b_or_c.c

Using an intermediate class (Target) guarantees that there will only be one link from D to either B or C. Does that make sense? See model inheritance for more information.

In your database there will be tables for Target, B, C and D, but not A, because that was marked as abstract (instead, columns related to attributes on A will be present in Target and D).

[Warning: I have not actually tried this code - any corrections welcome!]

You could also do a generic relation http://docs.djangoproject.com/en/dev/ref/contrib/contenttypes/#id1 and check the types to constrain it to B or C when setting or saving. This is probably more work than figuring out the direct reference, but might feel cleaner.