Merge, attach or refresh?

merge_or_attach.groovy

import gotchas.*

// Alice gets the location and saves it to her session...

def alice = [:]
Location.withNewSession { session ->
  alice.location = session.get(Location, 1L)
  session.close()
}

// Bob gets the location and modifies it...

Location.withNewSession { session ->
  def location = session.get(Location, 1L)
  location.code = UUID.randomUUID()
  session.save(location)
  session.flush()
  session.close()
}

// Alice comes back, attaches her location, modifies it, and tries to save it...

Location.withNewSession { session ->

  // Option #1: alice.location = alice.location.merge()
  
  alice.location = session.merge(alice.location) //=> throws HibernateOptimisticLockingFailureException
  alice.location.code = UUID.randomUUID()
  session.save(alice.location)
  session.flush()
  
  // Option #2: alice.location.attach()
  
  session.lock(alice.location, org.hibernate.LockMode.NONE) //=> causes the same exception but gets thrown on call to "flush()"
  alice.location.code = UUID.randomUUID()
  session.save(alice.location)
  session.flush()
    
  // Option #3: alice.location.refresh()
  
  session.refresh(alice.location) //=> actually seems to work but Hibernate doc says: "It is inadvisable to use this to implement long-running sessions that span many business tasks".
  alice.location.code = UUID.randomUUID()
  session.save(alice.location)
  session.flush()

  // Option #4: alice.location = Location.get(alice.location.id)
  
  alice.location = session.get(Location, alice.location.id) //=> works.
  alice.location.code = UUID.randomUUID()
  session.save(alice.location)
  session.flush()
  
  // Option #5: Location.lock(alice.location.id)
  
  Location.withTransaction {
    alice.location = session.get(Location, alice.location.id, org.hibernate.LockMode.UPGRADE) //=> works.
    alice.location.code = UUID.randomUUID()
    session.save(alice.location)
    session.flush()
  }
}