django admin inline with abstract class and m2m field - django

Question

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 that inherits 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 try 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!

Answer 1

Is there any good reason why you use this?
class Meta:
abstract = True
If possible, drop it, and then rebuild your database.
It might be interesting for you to read the answers to this stackoverflow question about the difference between abstract models and regular inheritance.

Answer 2

Your model structure is terrible: a M2M relationship will build a relationship table for both Models you try to connect, just like "Through keyword" in M2M field
As I can see, you just want to build a M2M relation between Person and Model based on Group.
class Person(models.Model):
name = models.CharField(max_length=128)
class Group(models.Model):
name = models.CharField(max_length=128)
type = models.CharField(max_lenght=32) # The Type of Group (Public/Private/etc..)
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)