[Django]-ForeignKey to abstract class (generic relations)

For a start, use Django’s multi-table inheritance, rather than the abstract model you have currently.

Your code would then become:

from django.db import models

class Service(models.Model):
    port = models.PositiveIntegerField()

class SSHService(Service):
    username = models.CharField(max_length=64)
    pkey = models.TextField()   

class TelnetService(Service):
    username = models.CharField(max_length=64)
    password = models.CharField(max_length=64)

class GenericTcpService(Service):
    pass

class GenericUDPService(Service):
    pass

class Node(models.Model):
    name = models.CharField(max_length=64)
    # various fields                                                                                                                                
    services = models.ManyToManyField(Service)

On the database level, this will create a ‘service’ table, the rows of which will be linked via one to one relationships with separate tables for each child service.

The only difficulty with this approach is that when you do something like the following:

node = Node.objects.get(pk=node_id)

for service in node.services.all():
    # Do something with the service

The ‘service’ objects you access in the loop will be of the parent type.
If you know what child type these will have beforehand, you can just access the child class in the following way:

from django.core.exceptions import ObjectDoesNotExist

try:
    telnet_service = service.telnetservice
except (AttributeError, ObjectDoesNotExist):
    # You chose the wrong child type!
    telnet_service = None

If you don’t know the child type beforehand, it gets a bit trickier. There are a few hacky/messy solutions, including a ‘serviceType’ field on the parent model, but a better way, as Joe J mentioned, is to use a ‘subclassing queryset’. The InheritanceManager class from django-model-utils is probably the easiest to use. Read the documentation for it here, it’s a really nice little bit of code.