migration note on subqueries

[jvanasco]

Change-Id: Id33779cb126a700d9adebe5f08f9d6c085589db4

Under 2.0, calls to in_ and not_in no longer accept an explicit .subquery().

Passing a .subquery() will cause typing issues from MyPy AND will raise runtime warnings.

There was no note of this in the migration guide. This may be an effect of another change that is disclosed in the migration guide. If so, I suggest nesting this text (or improved text describing this) under that section for ease in discovery and migration.

Description

Added a note to the 2.0 migration guide.

Checklist

This pull request is:

• A documentation / typographical / small typing error fix
  • Good to go, no issue or tests are needed
• A short code fix
  • please include the issue number, and create an issue if none exists, which
    must include a complete example of the issue. one line code fixes without an
    issue and demonstration will not be accepted.
  • Please include: Fixes: #<issue number> in the commit message
  • please include tests. one line code fixes without tests will not be accepted.
• A new feature implementation
  • please include the issue number, and create an issue if none exists, which must
    include a complete example of how the feature would look.
  • Please include: Fixes: #<issue number> in the commit message
  • please include tests.

Have a nice day!

[zzzeek]

the migration to 2.0 is still using session.query() so is not fully migrated

[zzzeek]

for migration to 2.0 the select() construct, not session.query(), should be used. with select(), you'd want to call subquery() on that unless you are using it inside of in_(). so this note should also mention that this is specific to in_()/not_in()

[jvanasco]

I'm going to move stuff around and incorporate your info. I'll use github for a bit and try to avoid gerrit if you don't mind.

[jvanasco]

I waited a bit before revisiting, and think this is the approach. Ready for review.