ForeignKey on_delete functionality should traverse (cascade, ha ha) to the DB backend

[gcbirzan]

By default, django creates FKs without an explicit ON DELETE on the backend. This, if nothing else, should be configurable. If for no other reason that when using some transaction isolation modes, the error returned by the backend will be the wrong one during concurrent modifications:

Table b links to table a. You insert in table b a link to ID 1 in a, at the same time as you delete it from a.

With the default (at least in PostgreSQL), in REPEATABLE READ, the error is IntegrityError, which is wrong, since in fact you should just retry the transaction. In lower isolation levels, the FK is deleted (as expected), though, granted the on delete signals are not called.

[Marc Tamlyn]

This is likely related to the mailing list conversation ​https://docs.djangoproject.com/en/dev/ref/django-admin/#runserver-port-or-address-port and the ticket #21127. Basically the consensus seems to be that if we do push to the database it will be a new option to on_delete in the FK declaration. This is also likely to be bundled with the removal of a default behaviour for on_delete.

[anonymous]

I think the URL should be ​https://groups.google.com/forum/#!topic/django-developers/FJMoGgtYIX4

Anyway, it is, but this doesn't involve any default behaviour change, so it should be way easier.

[Tim Graham]

Duplicate of #21961