django admin inline with abstract class and m2m field

刚在stackoverflow上问的问题，大神们求救
http://stackoverflow.com/questions/20580815/django-admin-inline-with-abstract-class-and-m2m-field

This problem almost drives me crazy :(

I was trying to use the StackedInline in admin interface.
The code below is in django documentation.

model.py

class Person(models.Model):
    name = models.CharField(max_length=128)

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person, through='Membership')

class Membership(models.Model):
    person = models.ForeignKey(Person)
    group = models.ForeignKey(Group)
    date_joined = models.DateField()
    invite_reason = models.CharField(max_length=64)

admin.py

class MembershipInline(admin.StackedInline):
    model = Membership
    extra = 1

class PersonAdmin(admin.ModelAdmin):
    inlines = (MembershipInline,)

class GroupAdmin(admin.ModelAdmin):
    inlines = (MembershipInline,)

But if the Group is an abstract base class and PublicGroup is subclass inherited from Group. Membership is used to relate PublicGroup and Person.

class Person(models.Model):
    name = models.CharField(max_length=128)

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person, through='%(class)s_Membership')

    class Meta:
        abstract = True

class PublicGroup(Group):
    pass

class Membership(models.Model):
    person = models.ForeignKey(Person)
    group = models.ForeignKey(Group)
    date_joined = models.DateField()
    invite_reason = models.CharField(max_length=64)

after running the command

python manage.py sql test

I got error "AssertionError: ForeignKey cannot define a relation with abstract class Group".
After searching for solution, I know foreign key cannot point to a abstract class. Some solutions recommended to use generic relation. So I change the code again.

class Person(models.Model):
    name = models.CharField(max_length=128)

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = generic.GenericRelation('Membership')

    class Meta:
        abstract = True

class PublicGroup(Group):
    pass

class Membership(models.Model):
    person = models.ForeignKey(Person)
    content_type = models.ForieignKey(ContentType)
    object_id = models.PositiveIntegerField()
    content_object = generic.GenericForeignKey()
    date_joined = models.DateField()
    invite_reason = models.CharField(max_length=64)

This time the command

python manage.py sql test

returns no error. But I got error when I tried to add data on admin interface. The error says Membership is not a foreign key of PublicGroup. StackedInline still doesn't work.

Now I really don't know what to do. Does anyone know how to achieve this function.
Thanks for reading!