What is the difference between a Multi-table inherited model and a simple One-to-one relationship between the same two models?

Question

What is the differences between these implementations? What does Django differently (beside inheriting Meta ordering and get_latest_by attribute) ?

Answer 1

1. You don't really get any python inheritance, that is you can't inherit/override methods or attributes from the model class Place in your class Restaurant:

For instance:

class Place(models.Model):
    name = models.CharField(max_length=50)

    def get_x(self):
        return 'x'

class Restaurant(models.Model):
    place = models.OneToOneField(Place)
    serves_pizza = models.BooleanField()

a_restaurant = Restaurant()
a_restaurant.get_x() # -> wouldn't work

This means that to obtain the name of a restaurant you can't do a_restaurant.name, you would need to follow the link: a_restaurant.place.name

Also note that when querying a Place object with the related Restaurant.

a_restaurant.save()
Place.objects.get(pk=a_restaurant.pk)  # won't work

You would have to write:

a_restaurant.save()
Place.objects.get(restaurant__pk=a_restaurant.pk)

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2 and 3. are almost the same. You do get real python inheritance with these.

a_restaurant = Restaurant()
a_restaurant.get_x() # would actually work and print 'x'

Your model class Restaurant inherits everything from Place: model fields, normal instance/class attributes, managers, methods... and you can also override almost anything of these: You can't override field attributes, that's not supported.

So now you can get the values of the fields from the parent model directly:a_restaurant.name because they are inherited.

Since with these implementation a Restaurant is a also Place you can query for a Place object with Restaurant data:

a_restaurant.save()
the_place = Place.objects.get(pk=a_restaurant.pk)  
# ^ this works now and returns the equivalent `Place` instance.
the_same_restaurant = the_place.restaurant

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The difference between 2 and 3 is easier to see if you give a different name to the field:

class Place(models.Model):
    name = models.CharField(max_length=50)

class Restaurant(Place):
    where = models.OneToOneField(Place, parent_link=True)
    serves_pizza = models.BooleanField()

Works exactly the same but to obtain the parent place of a Restaurant the attribute name is where:

the_place = a_restaurant.where

with 2 would have been:

the_place = a_restaurant.place_ptr

These means that place = models.OneToOneField(Place, parent_link=True) will only change the name of the link to the parent model instance. The default name is '{lowercase_model_name}_ptr'.

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Last example:

With 1:

place1 = Place.objects.create(name='place_1')
place2 = Place.objects.create(name='place_2')
restaurant1 = Restaurant.objects.create(place=place1, serves_pizza=True)

print Place.objects.all() # prints [place1, place2]
print Restaurant.objects.all() # prints [restaurant1]

With 2-3:

place1 = Place.objects.create(name='place_1')
place2 = Place.objects.create(name='place_2')
restaurant1 = Restaurant.objects.create(name='place_3', serves_pizza=True)

print Place.objects.all() # prints [place1, place2, place3]
print Restaurant.objects.all() # prints [restaurant1]

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Hope these helps. It grow a bit too long :/