Question

The documentation for sqlalchemy says that you can specify “FOR UPDATE NOWAIT” in PostgreSQL 8.1 upwards using Query.with_lockmode(‘update_nowait’). Does anyone know how to get it to add the FOR UPDATE NOWAIT instead of just FOR UPDATE?

PostgreSQL 9.1.6

query = db.session.query(MyTable)\
    .filter_by(process_status="PENDING")\
    .order_by(MyTable.id)\
    .with_lockmode('update_nowait')\

print query

sql:

SELECT 
MyTable.id AS MyTable_id
,MyTable.created_on AS MyTable_created_on
FROM MyTable 
WHERE MyTable.process_status = :process_status_1 
ORDER BY MyTable.id 
FOR UPDATE

Answer 1

I was able to reproduce similar case with Oracle 11g and SQLAlchemy 0.8.0b2.

db = create_engine('...',echo=True)
...
q = session.query(CauseCode.__table__).with_lockmode('update_nowait')
print q
>>>SELECT ... FROM cause_code FOR UPDATE
print renderquery(q)
>>>SELECT ... FROM cause_code FOR UPDATE NOWAIT
q.all()
>>>2013-01-23 09:58:12,665 INFO sqlalchemy.engine.base.Engine SELECT ... FROM cause_code FOR UPDATE NOWAIT

Usually default query renderer used by str() produce query different from the actual query executed to DB.
I suspect that sqlalchemy behave in the same way for Postgres in your case – print q produce dialect agnostic query.

PS renderquery() method implementation can be found here