This is an elegant solution that I quite appreciate because it uses zero data bytes:
some_flag CHAR(0) DEFAULT NULL
To set it to true, set
some_flag = ''and to set it to false, set
some_flag = NULL.
Then to test for true, check if some_flag
IS NOT NULL, and to test for false, check if some_flag
(This method is described in "High Performance MySQL: Optimization, Backups, Replication, and More" by Jon Warren Lentz, Baron Schwartz and Arjen Lentz.)
I can't say it is good for ORMs though.