Run database checks by default in some scenarios

[Adam Johnson]

#31055 disabled database checks by default in order to make tests that don't set up certain databases pass. This is great but it leads to a couple problems:

• Even with manage.py check --deploy as we suggest in the deployment checklist, the database configuration checks won't be run now.
• Users need to name all their database aliases on the command line if running manage.py check. Even if --database default was added to a deployment script early in the project, it's likely to be forgotten when adding a new database connection.
• Database checks *are* run during manage.py migrate, but that output can be long if several migrations are executed with a higher verbosity level, so any non-error messages are likely to be missed.

I write this as I'm trying to figure out what to do with the ​Django-MySQL database checks which show several warnings to users to improve their database configuration. I now realize that they will mostly be disabled for users on Django 3.1.

My suggestions would be:

• Enable --database __all__ to allow specifying all the database aliases on the command line.
• Make --deploy imply --database __all__ . Since this is normally run around the same time as migrate, it shouldn't be an issue to connect to all databases at that point (and was the behaviour of Django <3.1 anyway).
• Make runserver run database checks for all connections. This will help catch issues in development.

[Simon Charette]

Enable --database all to allow specifying all the database aliases on the command line.

I wonder if we could make --database without a value alias to --database __all__ instead. Maybe it's not a good idea due as might cause an issue similar to #31621?

Make --deploy imply --database all . Since this is normally run around the same time as migrate, it shouldn't be an issue to connect to all databases at that point (and was the behaviour of Django <3.1 anyway).
Make runserver run database checks for all connections. This will help catch issues in development.

Makes sense to me.