ActiveRecord and Threads

I do not want to set the concurrency flag in activerecord to true for
a variety of reasons. My ROR appln has a background thread spawned
which accesses the DB and send out an email. When this background
thread is accessing the DB, if a web request comes in ( that calls for
DB access) then we have 2 threads on the same ActiveRecord connection.
I dont mind synchronizing the access to the DB. . But not able to find
where I set that flag...

From a different perspective, I guess Mysql JDBC driver synchronises
the packet sending over TCP connection, so even if multiple threads
use the same connection in JAVA you are still safe ( nothing to do in
application code).

How do I get a similar functionality in ROR.?