Following up on #17338 ([nonrel] supports_deleting_related_objects
database feature flag):
carljm said that:
> [Django] is (sort of) _emulating_ SQL cascading deletes, but it does so in a
> way that doesn't assume anything at all from the backend.
I'm not sure this is correct. If I define the following models:
from django.db import models
class Person(models.Model):
name = models.CharField(max_length=100)
class Car(models.Model):
name = models.CharField(max_length=100)
owner = models.ForeignKey(Person)
Related objects are indeed deleted on a sqlite Django project:
In [1]: from vortaro.models import Person, Car
In [2]: p = Person.objects.create(name='bob')
In [3]: c = Car.objects.create(name='robin reliant', owner=p)
In [4]: p.delete()
In [5]: Car.objects.all()
Out[5]: []
In [6]: from django.db import connection
In [7]: connection.queries
Out[7]:
[{'sql': u'INSERT INTO "vortaro_person" ("name") VALUES (bob)',
'time': '0.001'},
{'sql': u'INSERT INTO "vortaro_car" ("name", "owner_id") VALUES
(robin reliant, 1)',
'time': '0.001'},
{'sql': u'SELECT "vortaro_car"."id", "vortaro_car"."name",
"vortaro_car"."owner_id" FROM "vortaro_car" WHERE
"vortaro_car"."owner_id" IN (1)',
'time': '0.001'},
{'sql': u'DELETE FROM "vortaro_car" WHERE "id" IN (1)', 'time':
'0.000'},
{'sql': u'DELETE FROM "vortaro_person" WHERE "id" IN (1)', 'time':
'0.000'},
{'sql': u'SELECT "vortaro_car"."id", "vortaro_car"."name",
"vortaro_car"."owner_id" FROM "vortaro_car" LIMIT 21',
'time': '0.000'}]
This isn't making a second query to deleting the Car model, it is
depending on the database to do this. For comparison, here's the same
on App Engine:
In [1]: from test_app.models import Person, Car
In [2]: p = Person.objects.create(name='bob')
In [3]: c = Car.objects.create(name='robin reliant', owner=p)
In [4]: p.delete()
In [5]: Car.objects.all()
Out[5]: [<Car: Car object>]
In [6]: c = Car.objects.get()
In [7]: c.owner
---------------------------------------------------------------------------
DoesNotExist Traceback (most recent call
last)
/home/wilfred/bleeding_edge/django-testapp/<ipython-input-7-
f94c13ea1b0a> in <module>()
----> 1 c.owner
/home/wilfred/bleeding_edge/django-testapp/django/db/models/fields/
related.pyc in __get__(self, instance, instance_type)
313 rel_obj = rel_mgr.using(db).get(**params)
314 else:
--> 315 rel_obj =
QuerySet(self.field.rel.to).using(db).get(**params)
316 setattr(instance, cache_name, rel_obj)
317 return rel_obj
/home/wilfred/bleeding_edge/django-testapp/django/db/models/query.pyc
in get(self, *args, **kwargs)
349 if not num:
350 raise self.model.DoesNotExist("%s matching query
does not exist."
--> 351 % self.model._meta.object_name)
352 raise self.model.MultipleObjectsReturned("get()
returned more than one %s -- it returned %s! Lookup parameters were
%s"
353 % (self.model._meta.object_name, num, kwargs))
DoesNotExist: Person matching query does not exist.
Have I horribly misunderstood?
Wilfred
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