On Dec 12, 4:53 pm, Wilfred Hughes <wilf...@potatolondon.com> wrote:
> > [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:
<SNIP>
> 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)
At this point you have a person, and a related car in DB.
> In [4]: p.delete()
>
> In [5]: Car.objects.all()
> Out[5]: []
<SNIP>
Delete starts here:
> {'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'},
We are deleting person with id 1. Here we fetch related cars.
> {'sql': u'DELETE FROM "vortaro_car" WHERE "id" IN (1)', 'time':
> '0.000'},
Now this is the emulated cascade. Above we found one car with id 1.
Now we are deleting it.
> {'sql': u'DELETE FROM "vortaro_person" WHERE "id" IN (1)', 'time':
> '0.000'},
All related objects have been deleted, so now we delete the person.
> {'sql': u'SELECT "vortaro_car"."id", "vortaro_car"."name",
> "vortaro_car"."owner_id" FROM "vortaro_car" LIMIT 21',
> 'time': '0.000'}]
Here nothing is found, as excepted.
> This isn't making a second query to deleting the Car model, it is
> depending on the database to do this.
It did make a delete to the Car model, but _before_ the person was
deleted. That is the correct order to delete the objects.
> For comparison, here's the same on App Engine:
<SNIP>
> In [7]: c.owner
<SNIP>
> DoesNotExist: Person matching query does not exist.
I believe AppEngine could have executed the above queries, there are
no joins. Instead it has deleted the Person, but left the related car
in the DB.
- Anssi
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