How to do a CAS UPDATE on single column CF?

[Blair Zajac]

How does one do an atomic update in a column family with a single column?
I have a this CF

  CREATE TABLE schema_migrations (
    version TEXT PRIMARY KEY,
  ) WITH COMPACTION = {'class': 'LeveledCompactionStrategy'};

that records which database migrations have been applied.  I want to do 
a CAS UPDATE to add a dummy lock token to prevent multiple migrations 
from running, but these three attempts fail (using the Python cql client):


>>> cursor.execute("UPDATE schema_migrations SET version = 'locked' 
WHERE version = 'locked' IF NOT EXISTS")
cql.apivalues.ProgrammingError: Bad Request: PRIMARY KEY part version 
found in SET part


>>> cursor.execute("UPDATE schema_migrations SET WHERE version = 
'locked' IF NOT EXISTS")
cql.apivalues.ProgrammingError: Bad Request: line 1:29 no viable 
alternative at input 'WHERE'



>>> cursor.execute("INSERT INTO schema_migrations (version) VALUES 
('locked') IF NOT EXISTS")
cql.apivalues.ProgrammingError: Bad Request: line 1:58 missing EOF at 'IF'

Thanks,
Blair