Add tables/ORM classes for streamflow orders and results

[rod-glover]

Resolves #72

Tasks:

• Add ORM classes
• Add tests for ORM classes
• Create migration
• Add tests for migration (existing tests cover this)

[rod-glover]

When reviewing, it might be worth your time to look at tests test_streamflow_workflow_new and test_streamflow_workflow_existing, which follow a happy-path sketch of the intended backend process when we eventually implement it. LMK if you see any issues here or any need for additional tests.

Otherwise this is pretty straightforward.

[corviday]

Do you have a new ORM diagram? If so, could it be added to this PR? I personally find the diagrams really helpful, hence my continual pestering about them. :)

[corviday]

Looks good to me. I appreciate the tests especially. I also ran the upgrade and downgrade on a dockerized database being used by an API, and verified that unsurprisingly, nothing broke.

EDIT: small issue with the downgrade I didn't catch at first, see the comment below.

[corviday]

This downgrade does not cleanly remove the upgrade, because it does not delete the streamflow_order_statuses type. While it's unlikely to be something we need to do, we can't run the upgrade, run the downgrade, and then run the upgrade again without this error message:

sqlalchemy.exc.ProgrammingError: (psycopg2.ProgrammingError) type "streamflow_result_statuses" already exists
 [SQL: "CREATE TYPE streamflow_result_statuses AS ENUM ('queued', 'processing', 'error', 'cancelled', 'ready', 'removed')"] (Background on this error at: http://sqlalche.me/e/f405)

Is it possible to delete the type? DROP TYPE or something?

[jameshiebert]

https://www.postgresql.org/docs/9.5/static/sql-droptype.html ?

[rod-glover]

Good catch, @corviday !

Hmm. According to SQLAlchemy docs, "The table.drop() call will now emit a DROP TYPE for a table-level enumerated type."

Several things are possible here:

1. You're running the tests with a pre-1.1.0 version of SQLAlchemy.
2. The ENUM isn't table-level. This seems the likeliest to me, but needs some research.
3. The ENUM is table-level, but Alembic is doing something that isn't exactly table.drop() -- hard to imagine that, but...