Sites framework post_migrate signal overwrites user created site.

[Tom Carrick]

There is some advice in the documentation:

django.contrib.sites registers a post_migrate signal handler which creates a default site named example.com with the domain example.com. This site will also be created after Django creates the test database. To set the correct name and domain for your project, you can use a data migration.

This advice can be quite contradictory. It only works if you've already run migrations once before adding the data migration, so it doesn't work for new deployments. This is because creating the default site will overwrite what is currently there, as the post_migrate signal runs after the user's data migration.

I think that create_default_site() shouldn't overwrite any data, it should only create the site if the table is empty, or at least only if the id isn't already in use.

If the current behaviour is intentional for some reason, I think we should at least remove the advice to create a data migration from the docs.

[Carlton Gibson]

Hi Tom.

Can I ask you to clarify here? AFAICS, ​the site creation is already behind a `not exists` conditional — so if you do create a data migration the create_default_site() is a noop.

Maybe I've misunderstood though...
Thanks.

[Tom Carrick]

You are right. Looking again, my data migration was wrong: it was assuming the site was already there.