Doing pre-migrate database requirements

[Kenial Sookyum Lee]

Some database backends has special requirements when the database gets migrated.

For example, when using django.contrib.gis.db.backends.spatialite as a backend, the database file should be initialized at first time with this command: "SELECT InitSpatialMetaData();" Unless, it causes some error like https://code.djangoproject.com/ticket/20968.

I guess this would be appropriate: adding a before_migration() method to DatabaseWrapper, and letting database backends inherit it to do some jobs.

New workaround codes are added to django/contrib/gis/apps.py. (Thanks @aaugustin!)

Pull request link: ​https://github.com/django/django/pull/3695

[Aymeric Augustin]

Look at django/contrib/postgres/apps.py for how to implement this.

[Claude Paroz]

Seems we have parallely worked on the same issue with two different approaches (#20968 reopened with ​https://github.com/django/django/pull/3697). Should we close this ticket now?

[Kenial Sookyum Lee]

Perhaps ... if InitSpatialMetaData query is executed in init_connection_state() method, then it will check it every time when opening a connection. As you know, there is no connection pooling on sqlite3 (and SpatiaLite) by default. It means that every query will execute PRAGMA table_info(geometry_columns);, I don't think it's a right way.

How about merging document stuff from pull request #3697 into mine? I forgot to update document about this.

[Kenial Sookyum Lee]

Replying to claudep:
Okay claudep, I will close this ticket. But as I commented, there is a possible issue on your code, so I'm gonna move my opinion on your ticket.