calculate_resource_count broken in sqlalchemy 1.4

[Brian Rosmaita]

Observed in master (xena development) with both 1.4.13 (proposed upper-constraint [0]) and 1.4.15 (most recent sqlalchemy release [1]), using oslo.db===8.6.0
[0] https://review.opendev.org/c/openstack/requirements/+/788339/7/upper-constraints.txt#150
[1] https://www.sqlalchemy.org/blog/2021/05/11/sqlalchemy-1.4.15-released

Error is: "sqlalchemy.exc.ArgumentError: Query has only expression-based entities"

12 of these in current tests with 3 variations:
sqlalchemy.exc.ArgumentError: Query has only expression-based entities - can't find property named "backup_metadata".
sqlalchemy.exc.ArgumentError: Query has only expression-based entities - can't find property named "snapshot_metadata".
sqlalchemy.exc.ArgumentError: Query has only expression-based entities - can't find property named "volume_metadata".

Think they're coming from here:
    File "/home/brosmait/repos/openstack/cinder/cinder/db/sqlalchemy/api.py", line 2387, in calculate_resource_count
    return query.with_entities(func.count()).scalar()

example failure:
cinder.tests.unit.api.v3.test_volumes.VolumeApiTest.test_list_volume_with_count_param_3

full test output: http://paste.openstack.org/show/805237/

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/cinder/+/790797

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/cinder/+/790797
Committed: https://opendev.org/openstack/cinder/commit/a3682eac340c48b0b7430443d52bf7af5af5cf86
Submitter: "Zuul (22348)"
Branch: master

commit a3682eac340c48b0b7430443d52bf7af5af5cf86
Author: Gorka Eguileor <email address hidden>
Date: Tue May 11 17:59:46 2021 +0200

    SQLA 1.4: Fix calculate_resource_count

    SQLAlchemy 1.4 is failing on method calculate_resource_count with error:

      sqlalchemy.exc.ArgumentError: Query has only expression-based entities
      - can't find property named "XYZ_metadata".

    Where XYZ is backup, snapshot, and volume.

    This is most likely caused by SQLAlchemy not knowing what to do with the
    joined loads once we replace the query using `with_entities`.

    This patch fixes this issues by not doing unnecessary joined loads when
    counting resources.

    Closes-Bug: #1928085
    Change-Id: I6b60106347aede024e5546f766453b4fec648cc2

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/cinder 19.0.0.0b1 development milestone.