title: Firebase preventing two different connections updating same record
body: <p>If I have an event app that reads the db and finds an open event slot and presents a dialog to the user do book it, how do I prevent a second user also opening up the same object and booking it as well?  I can't find mention of a semaphore lock on the db for a record that would prohibit the second connection from making an update to the same record (in rejecting it because the flag on that record is now set to true).</p>