Add docs and example of using plain SQLAlchemy models with Flask-SQLAlchemy

[e271828-]

There are many examples of people struggling to work around this limitation in flask-sqlalchemy, including various issues and PRs like:

#269
#250
#629

This PR documents a simple solution to bi-directional compatibility.

[ThiefMaster]

I don't think runtime switching makes much sense for something like this...

[e271828-]

I don't think runtime switching makes much sense for something like this...

The obvious alternative is refactoring initialization to make model_class apply to generic models instead of only working properly (e.g. attaching query class) when flask_sqlalchemy.Model is subclassed.

Seems like people have been proposing patches to do that for several years, none of which have landed.

What would you suggest?

[davidism]

Closing for similar reasons to #629. This also seems overly complex. You can already use regular models with db.session.query(), and you can use Flask-SQLAlchemy outside a web app by doing app.app_context().push() first.