Move `Manager.raw()` and `Manager._insert()` to the QuerySet class.

[loic84]

After the introduction of #20625, most methods that involve a DB query have been removed from Manager and are automatically proxied to QuerySet, only raw() and _insert() remain. After discussion with @akaariai on IRC we came to the conclusion that it would make sense to move these to the QuerySet class as well.

The previous goal was for QuerySet to focus on retrieval, but this has somewhat failed as most methods that could belong to Manager (like create()) already live on the QuerySet class. The introduction of hybrid methods like get_or_create() or update_or_create() also demonstrates that the distinction between the different kinds of queries is not obvious to make.

If this ticket is accepted, the new objective would be defined as follows:

• Anything "query" belongs to QuerySet.
• Manager is responsible for the integration between a Model and a QuerySet.
• Manager works as a hook to pre/post-process or even replace QuerySet methods when needed. (See current implementation of Manager.all() or RelatedManager.create()).

[loic84]

PR ​https://github.com/django/django/pull/1411

[Tim Graham]

Assuming the idea isn't rejected by someone else, the patch looks good.

[Anssi Kääriäinen]

I think the move makes sense. .raw() belongs to queryset just as much as .create(). So, things will be a bit more consistent after the move.

[Tim Graham <timograham@…>]

In acd1d439fd9b3e77bc0291dcd62c09f345d8622c:

Fixed #20826 -- Moved Manager.raw() and Manager._insert() to the QuerySet class.