module ActiveRecord::Locking::Pessimistic

def lock!(lock = true)

the locked record.
or pass true for "FOR UPDATE" (the default, an exclusive row lock). Returns
lock. Pass an SQL locking clause to append the end of the SELECT statement
Obtain a row lock on this record. Reloads the record to obtain the requested 
def lock!(lock = true)
  reload(:lock => lock) if persisted?
  self
end

def with_lock(lock = true)

as argument (see lock!).
before yielding. You can pass the SQL locking clause
Wraps the passed block in a transaction, locking the object 
def with_lock(lock = true)
  transaction do
    lock!(lock)
    yield
  end
end