# Django ORM does not support inheritance. Inheritance is a # good thing. Often I want some of the database models to # share some common fields and the same manager. For # example consider the case of adding the delete proxy. class RetainDeletesMixin(object): class NonDeleted(models.Manager): def get_query_set(self): return super(NonDeleted, self).get_query_set().filter( delete_flag=0) delete_flag = models.IntegerField(null=True, blank=True, default=0) objects = NonDeleted() def delete(self): self.delete_flag = 1 self.save() class Item(RetainDeletesMixin, models.Model): ... # Here we are extending the model Item by adding a mixin # that overrides the delete method - so that when we do # item.delete() the model is not actually deleted but # merely flagged (delete_flag=1) so. This means the # get_query_set method should also be told not to return # the flagged models. In Python speak, # >>> items = Item.objects.all() # >>> item = items[0] # >>> len(items) # 1 # >>> item.delete_flag # 0 # >>> item.delete() # >>> item.delete_flag # 1 # >>> len(Item.objects.all()) # 0 # But this will not work. Django does not consider # delete_flag, which is inherited from RetainDeletesMixin, # as a database column at all. So I came up with the # following metaclass hack which enables you to design # the django models based on the inheritance pattern like # the above, class RetainDeletesMixin(object): class NonDeleted(models.Manager): def get_query_set(self): # Ideally, we should be using `super` here but we don't because # the variable `NonDeleted` will not be accessible once we # 'copy' this class in the metaclass. return models.Manager.get_query_set(self).filter( delete_flag=0) delete_flag = models.IntegerField(null=True, blank=True, default=0) objects = NonDeleted() def delete(self): self.delete_flag = 1 self.save() def django_extends(base): class ProxyMetaClass(models.Model.__metaclass__): def __new__(cls, name, bases, attrs): # The following attributes must be moved *prior* to deferring # execution to the models.Model's metaclass, because they # will be manipulated by __new__ # - models.fields.Field instances # - objects (ModelManager) for key in dir(base): if not key.startswith('_'): obj = getattr(base, key) if isinstance(obj, models.fields.Field) or \ key == 'objects': attrs[key] = obj delattr(base, key) # Delete objects that have attribute 'contribute_to_class' # for otherwise that will break in # base.py:Model.add_to_class # Eg: inner classes inherited from models.Manager elif hasattr(obj, 'contribute_to_class'): delattr(base, key) return super(ProxyMetaClass, cls).__new__(cls, name, tuple([base]+list(bases)), attrs) frame = sys._getframe(1) frame.f_locals['__metaclass__'] = ProxyMetaClass class Item(models.Model): django_extends(RetainDeletesMixin) ... # What this basically does is - copy the database fields # and objects from the base class (RetainDeletesMixin) to # the derived class (Item) so that Django will recognize it # upon processing in ModelBase. It also makes # RetainDeletesMixin a base class of Item. I have not # tested this code extensively, but it works for the models # that I have written in our application.