Add support for multiple databases migration

[ncloudioj]

This PR is aimed to partially fix #20, it uses "multidb" template of Alembic. Flask-Migrate users, however, still need to change the alembic.ini and env.py to provide the required database meta information.

[miguelgrinberg]

Thanks. I think I'm going to be able to make the edits to the config files automatic, all on top of your patch. I'll delay the merge a couple days until I can have that fully figured out.

[miguelgrinberg]

@ncloudioj any reason why you did not follow the recommended format for binds in the test application? You created two sqlalchemy objects, instead of using a single one and assigning tables to each bind with the bind_key argument.

[ncloudioj]

@miguelgrinberg It has to do with the "metadata" in "env.py". Again, have tried reusing a single db instance, but Alembic only created schema for the main db. Seems like Alembic needs different "metadata" for different database.

[miguelgrinberg]

Unfortunately I think that is going to be a deal killer for some. Is it even possible to set up a relationship across databases with this model?

In any case, I don't remember the exact structure, but Flask-SQLAlchemy keeps the databases and their metadata internally, so I think it should be possible to get them using the recommended configuration.

[ncloudioj]

Yes, I agree that it's annoying. But unfortunately Alembic needs metadata for each db engine. Well, will take another look to see if any improvement exists.

[miguelgrinberg]

@ncloudioj I'll do some research on this as well. SQLAlchemy has the support for binds, so we need to figure out how Alembic can use that.

[miguelgrinberg]

@ncloudioj I'm merging your patch, and right after I'm putting my changes to make it fully configuration free. Thanks!

[ncloudioj]

@miguelgrinberg Awesome! Apparently, way more reasonable than my hacky one. Well done.

We recently introduced another database in production. Also, we'd like to keep the amazing Flask-Migrate as our database migration tool. Finally, we can have both. Thanks.

[miguelgrinberg]

@ncloudioj Check the changes I've made to the test application. The way you configured the bind for the second table was incorrect, and caused the table to go into the main database. That made me realize that the unit tests were insufficient for multidb, I needed to also ensure that the tables were going to the correct databases. All in all it is a great addition to the project, thanks!

[espositocode]

This is amazing. Thanks!

👍