The previous version of this page is obsolete due to the model syntax change. Additionally, there's now a better reference for this topic: http://www.djangoproject.com/documentation/models/subclassing/ = magic-removal: Model Inheritance = This is a proposal for how subclassing should work in Django. There a lot of details to get right, so this proposal should be very specific and detailed. Most of the ideas here come from the thread linked below: http://groups.google.com/group/django-developers/browse_frm/thread/ea5e0bf903058fac/9a68ac0d99cb6d7d?q=semantics&rnum=1#9a68ac0d99cb6d7d Here are some additional notes from Robert Wittams on allowing different storage models. http://groups.google.com/group/django-developers/browse_frm/thread/9a9a2cfacce690d/dfcd5a71ba523534?q=Just+a+few+vague+notes&rnum=1#dfcd5a71ba523534 == Subclassing == For subclassing, there are 3 main issues: 1. How do we model the relations in SQL? 2. How do joins work? 3. How does the API work? Note that I have only provided examples for single inheritance here. Is multiple inheritance worth supporting? The examples will use the following models: {{{ #!python class Place(models.Model): name = models.CharField(maxlength=50) class Restaurant(Place): description = models.TextField() class ItalianRestaurant(Restaurant): has_decent_gnocchi = models.BooleanField() }}} === 1. Modeling parent relations in SQL? === The general consesus seems to be this: {{{ #!sql CREATE TABLE "myapp_place" ( "id" integer NOT NULL PRIMARY KEY, "name" varchar(50) NOT NULL ); CREATE TABLE "myapp_restaurant" ( /* PRIMARY KEY REFERENCES "myapp_places" ("id") works for postgres, what about others? */ "id" integer NOT NULL PRIMARY KEY REFERENCES "myapp_places" ("id"), "description" text NOT NULL ); CREATE TABLE "myapp_italianrestaurant" ( "id" integer NOT NULL PRIMARY KEY REFERENCES "myapp_restaurant" ("id"), "has_decent_gnocchi" bool NOT NULL ); }}} === 2. Modeling joins in SQL === When we want a list of {{{ItalianRestaurant}}}s, we obviously need all the fields from myapp_restaurant and myapp_place as well. This could be accomplished by inner joins. It would look something like this: {{{ #!sql SELECT ... FROM myapp_italianrestaurant as ir INNER JOIN myapp_restaurant as r ON ir.restaurant_id=r.id INNER JOIN myapp_place as p ON r.place_id=p.id }}} But what if we want a list of {{{Place}}}s, what should we do? We can either ''just'' get the places: {{{ #!sql SELECT ... FROM myapp_place }}} Or we can get ''everything'' with left joins (this allows the iterator to return objects of the appropriate type, rather than just a bunch of {{{Places}}}): {{{ #!sql SELECT ... FROM myapp_place as p LEFT JOIN myapp_restaurant as r ON r.place_id=p.id LEFT JOIN myapp_italianrestaurant as ir ON ir.restaurant_id=r.id }}} Imagine we have more than one subclass of {{{Place}}} though. The join clause and the column list would get pretty hefty. This could obviously get unmanageable pretty quickly. '' I think some dbs have a maximum number of joins (something like 16), and even within the maximum, the query optimizer will either spend a while deciding which way to best join the tables or it will give up and choose the wrong way quickly. This wording is FUD-- I'll try to find specifics. --jdunck '' Another option is to lazily load objects like {{{Restaurant}}} and {{{ItalianRestaurant}}} while we're iterating over {{{Place.objects.all()}}}, but that requires a ''lot'' of database queries. Either way, doing this will be expensive, and api should reflect that. You're much better off ''just'' using {{{Place}}}s fields if you are going to iterate over {{{Place.objects.all()}}} === 3. API === The following API examples assume we have created these objects: {{{ #!python p = Place(name='My House') r = Restaurant(name='Road Kill Cafe', description='Yuck!') ir = ItalianRestaurant(name="Ristorante Miron", description="Italian's best mushrooms restaurant", has_decent_gnocchi=True) }}} For the following examples, assume {{{Place.objects.get(2)}}} returns {{{r}}} and {{{Place.objects.get(3)}}} returns {{{ir}}}. || || python || result || || A. || {{{Place.objects.count()}}} || 3 || || B. || {{{Restaurant.objects.count()}}} || 2 || || C. || {{{ItalianRestaurant.objects.count()}}} || 1 || || D. || {{{Place.objects.get(2).description}}} || 'Yuck!' or {{{AttributeError}}}? || || E. || {{{Restaurant.objects.get(2).description}}} || 'Yuck!' || == Change the current usage of subclassing == {{{ #!python class MyArticle(Article): ...fields... class META: module_name = 'my_articles' remove_fields = ...some fields... }}} would change to: {{{ #!python class MyArticle(meta.Model): ...fields... class META: copy_from = Article remove_fields = ...some fields... }}} == Ramblings on Magic Removal Subclassing == For the above Restaurant example: {{{ class Place(models.Model): name = models.CharField(maxlength=50) class Restaurant(Place): description = models.TextField() }}} we want Restaurant to have a 'name' CharField. However, fields from parent classes are not included when the class is created.