What is the difference between a Multi-table inherited model and a simple One-to-one relationship between the same two models?
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)
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
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'
.
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]
Hope these helps. It grow a bit too long :/
1 - to create restaurant you need to create place, after create restaurant, after link them,2 - then creating restaurant, new place created and linked automaticaly,3 - you renamed parent link to place.
Using Model Inheriting with Content Types you can list all Cafes, Restaurants, Bars, etc iterating on Place.objects.all()