How do you model this in django?

Answer 1

I think you should try:

class Door(models.Model):
    bar = models.CharField(max_length=123)

class House(models.Model):
    foo = models.CharField(max_length=123)
    door = models.ForeignKey(Door)

class Caravan(models.Model):
    foo = models.CharField(max_length=123)
    door = models.ForeignKey(Door)

Answer 2

class Door(models.Model):
    bar = models.CharField(max_length=123)

class Caravan(models.Model):
    foo = models.CharField(max_length=123)
    doors = models.ManyToManyField(Door)

class House(models.Model):
    foo = models.CharField(max_length=123)
    doors = models.ManyToManyField(Door)

Answer 3

Can you clarify what you're really looking for with an example query? It's not clear to me. This is what I think you're looking for:

class Door(models.Model):
  bar = models.CharField(max_length=123)

class House(Door):
  foo = models.CharField(max_length=123)

class Caravan(Door):
  foo = models.CharField(max_length=123)

Then you can do things like Caravan.objects.values('foo','bar')