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
