Here is an example of where that SAWarning comes from:

nova.tests.unit.api.openstack.compute.test_hosts.HostTestCaseV21.test_show_works_correctly
------------------------------------------------------------------------------------------

Captured pythonlogging:
~~~~~~~~~~~~~~~~~~~~~~~
    b'2019-01-31 17:48:15,014 INFO [248_add_expire_reservations_index] Skipped adding reservations_deleted_expire_idx because an equivalent index already exists.'
    b''

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    b'Traceback (most recent call last):'
    b'  File "/home/osboxes/git/nova/nova/tests/unit/api/openstack/compute/test_hosts.py", line 410, in test_show_works_correctly'
    b"    db.instance_destroy(ctxt, i_ref1['uuid'])"
    b'  File "/home/osboxes/git/nova/nova/db/api.py", line 753, in instance_destroy'
    b'    return IMPL.instance_destroy(context, instance_uuid, constraint)'
    b'  File "/home/osboxes/git/nova/nova/db/sqlalchemy/api.py", line 170, in wrapper'
    b'    return f(*args, **kwargs)'
    b'  File "/home/osboxes/git/nova/.tox/py35/lib/python3.5/site-packages/oslo_db/api.py", line 154, in wrapper'
    b'    ectxt.value = e.inner_exc'
    b'  File "/home/osboxes/git/nova/.tox/py35/lib/python3.5/site-packages/oslo_utils/excutils.py", line 220, in __exit__'
    b'    self.force_reraise()'
    b'  File "/home/osboxes/git/nova/.tox/py35/lib/python3.5/site-packages/oslo_utils/excutils.py", line 196, in force_reraise'
    b'    six.reraise(self.type_, self.value, self.tb)'
    b'  File "/home/osboxes/git/nova/.tox/py35/lib/python3.5/site-packages/six.py", line 693, in reraise'
    b'    raise value'
    b'  File "/home/osboxes/git/nova/.tox/py35/lib/python3.5/site-packages/oslo_db/api.py", line 142, in wrapper'
    b'    return f(*args, **kwargs)'
    b'  File "/home/osboxes/git/nova/nova/db/sqlalchemy/api.py", line 227, in wrapped'
    b'    return f(context, *args, **kwargs)'
    b'  File "/home/osboxes/git/nova/nova/db/sqlalchemy/api.py", line 1797, in instance_destroy'
    b'    count = query.soft_delete()'
    b'  File "/home/osboxes/git/nova/.tox/py35/lib/python3.5/site-packages/oslo_db/sqlalchemy/orm.py", line 32, in soft_delete'
    b'    synchronize_session=synchronize_session)'
    b'  File "/home/osboxes/git/nova/.tox/py35/lib/python3.5/site-packages/sqlalchemy/orm/query.py", line 3486, in update'
    b'    update_op.exec_()'
    b'  File "/home/osboxes/git/nova/.tox/py35/lib/python3.5/site-packages/sqlalchemy/orm/persistence.py", line 1334, in exec_'
    b'    self._do_pre_synchronize()'
    b'  File "/home/osboxes/git/nova/.tox/py35/lib/python3.5/site-packages/sqlalchemy/orm/persistence.py", line 1405, in _do_pre_synchronize'
    b'    self._additional_evaluators(evaluator_compiler)'
    b'  File "/home/osboxes/git/nova/.tox/py35/lib/python3.5/site-packages/sqlalchemy/orm/persistence.py", line 1543, in _additional_evaluators'
    b'    expression._literal_as_binds(value))'
    b'  File "/home/osboxes/git/nova/.tox/py35/lib/python3.5/site-packages/sqlalchemy/orm/evaluator.py", line 39, in process'
    b'    return meth(clause)'
    b'  File "/home/osboxes/git/nova/.tox/py35/lib/python3.5/site-packages/sqlalchemy/orm/evaluator.py", line 71, in visit_column'
    b'    "UPDATE / DELETE expressions." % clause)'
    b'  File "/home/osboxes/git/nova/.tox/py35/lib/python3.5/site-packages/sqlalchemy/util/langhelpers.py", line 1312, in warn'
    b'    warnings.warn(msg, exc.SAWarning, stacklevel=2)'
    b"sqlalchemy.exc.SAWarning: Evaluating non-mapped column expression 'updated_at' onto ORM instances; this is a deprecated use case.  Please make use of the actual mapped columns in ORM-evaluated UPDATE / DELETE expressions."
    b''