code: def acquire(self, *args, **kwargs):
          
          if not self.field.lockable:
              return
          if self.already_locked_by_model:
              self.sub_lock_mode = True
              return
          self.already_locked_by_model = True
          super(FieldLock, self).acquire(*args, **kwargs)
docs: Really acquire the lock only if it's not a sub-lock. Then save the
          sub-lock status.