How to use merge - jpa

Question

In my Web tier I show a list of Tasks and for each task it shows how many TaskErrors it has. The tasks are fetched in a controller class this way:
public List<Task> getTasks () {
return taskEJB.findAll();
}
Task has a #OneToMany mapping with TaskErrors: a task can have multiple task errors.
When I delete a single TaskError, a controller in the Web tier calls TaskErrorFacade:remove(taskError). TaskErrorFacade is a stateless session bean. The code:
public void remove (TaskError taskError) {
entityManager.remove(entityManager.merge(taskError));
Task task = taskError.getTask();
task.deleteTaskError(taskError);
task = entityManager.merge(task);
}
Task:deleteTaskErrors(TaskError taskError) simply deletes the task from the List:
public void deleteTaskError (TaskError p_error) {
taskErrorCollection.remove(p_error); // gives true
}
This works. The Web tier shows a tabel with all the task and the number of Task Errors it has. The number drops by 1 after deleting a TaskError.
But... the following two implementations of TaskErrorFacade:remove(taskError) don't work: the tabel in the Web tier doesn't subtract 1 of the total Task Error count (however, they are deleted in the database):
public void remove (TaskError taskError) {
entityManager.remove(entityManager.merge(taskError));
Task task = taskError.getTask();
task = entityManager.merge(task); // the merge is done before the deletion
task.deleteTaskError(taskError);
}
And:
public void remove (TaskError taskError) {
entityManager.remove(entityManager.merge(taskError));
taskError.getTask().deleteTaskError(taskError); // no merge
}
Why are the two implementations above not working?
Edit: also important are the mappings of the relations. Task is mapped in TaskError this way:
#JoinColumn(name = "task_id", referencedColumnName = "id")
#ManyToOne(optional = false)
private Task task;