user_one = User.find(1)
user_two = User.find(1)

user_one.name = "John"
user_one.save
# Run at the same instance 
user_two.name = "Doe"
user_two.save # Raises a ActiveRecord::StaleObjectError

appointment = Appointment.find(5)
appointment.lock!
#no other users can read this appointment, 
#they have to wait until the lock is released
appointment.save! 
#lock is released, other users can read this account