Grails pessimistic locking

Question

I'm trying to permform a lock on an entity inside a service in Grails.

The code:

TheEntity entityForUpdate = TheEntity.lock(entityId)

(...) // some code

entityForUpdate.save()

This code works until a concurrent acces ocurs. When it happens, one thread remains locked while the first is performing the code. Once the lock is released an exception is thrown:

ERROR errors.GrailsExceptionResolver  - StaleObjectStateException occurred when processing request: [POST] /Portal/Controler/theMethod - parameters:
param1: 18
param2: 86,32

Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect): [core.TheEntity#133]. Stacktrace follows:
Message: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect): [core.TheEntity#133]
Line | Method
->>   96 | lock                                  in org.hibernate.dialect.lock.SelectLockingStrategy
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
|   1441 | lock                                  in org.hibernate.persister.entity.AbstractEntityPersister
|    110 | upgradeLock . . . . . . . . . . . . . in org.hibernate.event.def.AbstractLockUpgradeEventListener
|     86 | onLock                                in org.hibernate.event.def.DefaultLockEventListener
|    774 | fireLock . . . . . . . . . . . . . .  in org.hibernate.impl.SessionImpl
|    758 | lock                                  in     ''
|    665 | doInHibernate . . . . . . . . . . . . in org.springframework.orm.hibernate3.HibernateTemplate$10
|    406 | doExecute                             in org.springframework.orm.hibernate3.HibernateTemplate
|    374 | executeWithNativeSession . . . . . .  in     ''
|    663 | lock                                  in     ''
|    148 | lock . . . . . . . . . . . . . . . .  in 
...

I don't undrestand why it happens. Why the lock method is throwing this exception?

EDIT:

I have finally chose to remove the version from the entity in order to avoid the problem:

class TheEntity {
  ...
  static mapping = {
    version false
  }
  ...
}

In order to get the updated data I have also set the transaction mode of the service method to READ_COMMITED

@Transactional(isolation=Isolation.READ_COMMITTED)
def theMethod() {

}