ForeignKey to abstract class (generic relations)

Question

I\'m building a personal project with Django, to train myself (because I love Django, but I miss skills). I have the basic requirements, I know Python, I carefully read the

Answer 1

An actual service can only be on one node, right? In that case when not have a field

node = models.ForeignKey('node', related_name='services')

in the service class?

Answer 2

If you are looking for generic foreign key relations you should check the Django contenttypes framework (built into Django). The docs pretty much explain how to use it and how to work with generic relations.

Answer 3

I think one approach that you might consider is a "subclassing queryset". Basically, it allows you to query the parent model and it will return instances of the child models in the result queryset. It would let you do queries like:

models.service.objects.all() 

and have it return to you results like the following:

[ <sshServiceInstance>, <telnetServiceInstance>, <telnetServiceInstance>, ...]

For some examples on how to do this, check out the links on the blog post linked below.

http://jazstudios.blogspot.com/2009/10/django-model-inheritance-with.html

However, if you use this approach, you shouldn't declare your service model as abstract as you do in the example. Granted, you will be introducing an extra join, but overall I've found the subclassing queryset to work pretty well for returning a mixed set of objects in a queryset.

Anyway, hope this helps, Joe

Answer 4

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.