Primary Key Violation on JPA save method - spring

Question

I'm facing an annoying problem, which I think it's really stupid.
I'm using an H2 database with JPA and Hibernate.
This is my save method on ScheduleService.class:
#Transactional(propagation = Propagation.REQUIRES_NEW)
public int save(Schedule sched) {
try {
scheduleRepository.save(sched);
} catch(Exception e) {...}
}
The sentence "scheduleRepository.save(sched)" is throwing the following exception:
org.springframework.dao.DataIntegrityViolationException: could not execute statement; SQL [n/a]; constraint ["UK_4R97H1MMVBOG9TD05OH9FKL16_INDEX_5 ON PUBLIC.SCHEDULE(CHANNEL, PROGRAMME, START, END) VALUES ( /* key:65574 */ null, TIMESTAMP '2015-09-15 17:00:00.0', null, TIMESTAMP '2015-09-15 16:40:00.0', 3, 13)"; SQL statement:
insert into schedule (channel, end, programme, published, start, idSched) values (?, ?, ?, ?, ?, ?) [23505-175]]; nested exception is org.hibernate.exception.ConstraintViolationException: could not execute statement
And printing to the output console the following message:
Unique index or primary key violation: "UK_4R97H1MMVBOG9TD05OH9FKL16_INDEX_5 ON PUBLIC.SCHEDULE(CHANNEL, PROGRAMME, START, END) VALUES ( /* key:65574 */ null, TIMESTAMP '2015-09-15 17:00:00.0', null, TIMESTAMP '2015-09-15 16:40:00.0', 3, 13)"; SQL statement:
insert into schedule (channel, end, programme, published, start, idSched) values (?, ?, ?, ?, ?, ?) [23505-175]
I've removed my #UniqueConstraint annotation in Schedule entity to make sure it is not a unique constraint problem.
The error persists, so it must be caused by a primary key violation.
This is how I define the primary key of Schedule entity:
#Id
#Column(name = "idSched", nullable = false)
#GeneratedValue(strategy = GenerationType.TABLE)
protected Long idSched;
It should be assigning a new unique idSched field for every row is inserted in the table, right?
What could be the problem which is causing the exception?
Thank you.
UPDATE:
Based on Bohuslav suggestion, I've changed the primary key declaration as this:
#Id
#TableGenerator(
name="scheduleGen",
table="schedule_sequence_table",
pkColumnValue="idSched",
allocationSize=1)
#GeneratedValue(strategy = GenerationType.TABLE, generator="scheduleGen")
#Column(name = "idSched", nullable = false)
protected Long idSched;
For the moment it seems to work fine.
UPDATE 2: Based on Sreenath comment:
I'm generating the tables only once (they're persisted as files on disk). Actually there was a unique key constraint on schedule table. However, since it was throwing that exception, I removed the constraint and removed database files to make sure the problem was not in the unique constraint but in the primary key instead. Even without the unique constraint and a brand new database, the exception still remained, so then I came with the #TableGenerator solution. Apparently that solved the problem.
But then I put my unique constraint again, removed database files so tables can be regenerated, and started and stopped the application a few times.
Now the problem has come again.
This is my unique constraint:
#Table(uniqueConstraints={#UniqueConstraint(columnNames = {"channel", "programme", "start", "end"})})
#Entity(name = "schedule")
public class Schedule { ...}
Of course, before I save an entity, I check if there is any other entity with the same unique members as the one I'm adding:
Schedule schedIn = schedRep.findOneByChannelAndProgrammeAndStartAndEnd(
sched.getChannel(), sched.getProgramme(), sched.getStart(), sched.getEnd());
// Check if there exists any schedule with the same {channel, programme, start, end} values
if (schedIn == null) {
// If not, then save it
schedRep.save(sched);
}
The method schedRep.findOneByChannelAndProgrammeAndStartAndEnd() is automatically generated by JPA. So that's weird, that query cannot find any schedule with the same {channel, programme, start, end} values, but when I try to save them, the unique constraint is violated.
Where could be the problem then?