Add on_update support

[Anssi Kääriäinen]

There are cases where on-update cascade support would be useful. Currently Django offers no way to cascade updates.

An experimental patch implementing on_update=models.CASCADE support for foreign keys is available from: ​https://github.com/akaariai/django/tree/update_cascade. It is based on composite foreign keys branch (​https://github.com/akaariai/django/tree/soc2013/composite-fields).

I'll attach a sample project to this ticket. Unpack it, use the above mentioned branch of Django, run syncdb, runserver and try in admin. Note that the country/city names are stored locally in the models, so country name update cascades to cities and from cities to streets.

[Anssi Kääriäinen]

Sample project

[loic84]

Awesome work.

I just had a brief look at the diff, it will need special handling to be compatible with migrations, my patch for #21275 addresses this issue.

[Akash Kumar Sen]

This seems to be a interesting topic to work on, but I am not sure about the ideal behavior of the on_update parameter. Some explanation would be very helpful. Are we talking about something similar to the SQL ON UPDATE clause here?