Disable SQLALCHEMY_DATABASE_URI when SQLALCHEMY_BINDS is used

[cykerway]

I don't see there is a need for SQLALCHEMY_DATABASE_URI when SQLALCHEMY_BINDS is used. If users don't set SQLALCHEMY_DATABASE_URI then it gets a default value of an in-memory sqlite database. This may confuse users if they forget to set __bind_key__ on a model. In that case, that model doesn't fail but writes into the in-memory database.

I was trying to set SQLALCHEMY_DATABASE_URI to None, but flask-sqlalchemy doesn't seem to allow that with db.create_all(), even if all the models have __bind_key__ set to a valid bind. Can you fix the plugin so that SQLALCHEMY_DATABASE_URI defaults to None, or at least let users set it to None? Thank you.

Possibly related to: #471, #472

[rsyring]

I do believe we should remove the default value for SQLALCHEMY_DATABASE_URI and instead throw an error when the value is None and binds are not being used. @davidism has a comment in #472 that indicates the current state of things is misleading.

Bumping this milestone to 3.0 since it would be a backwards incompatible change.

[lbeaufort]

Hi, I'd like to take on this issue :)

[lbeaufort]

Update - this change gets pretty messy if I branch of master before this PR is merged, so I was pausing on the work. I could put up a [WIP] PR if needed, possibly branching off the latest changes in #727 and rebasing/merging changes as needed.

[davidism]

Merged #727, go ahead with a PR!