insanity.py

from django.db import models

class DeviceA(object):
    def do_something(self, string='Hello'):
        print "Device A something '%s'" % string

    def do_something_else(self):
        print "Device A something else"


class DeviceB(object):
    def do_something(self, string='Hello'):
        print "Device B something '%s'" % string

    def do_something_else(self):
        print "Device B something else"


class DeviceC(object):
    def do_something(self, string='Hello'):
        print "Device C something '%s'" % string

    def do_something_else(self):
        print "Device C something else"


class Device(models.Model):
    _device = None
    DEVICE_TYPES = (
        ('A', 'Device A'),
        ('B', 'Device B'),
        ('C', 'Device C'),
    )
    type = models.CharField(max_length=1, choices=DEVICE_TYPES)

    @property
    def device(self):
        if self._device is None:
            if self.type == 'A':
                self._device = DeviceA()

            elif self.type == 'B':
                self._device = DeviceB()

            elif self.type == 'C':
                self._device = DeviceC()
        
        return self._device