CRUDRepository unable to save modified entities - spring

Question

I'm trying to fetch some data from the database, update a field with some other entity and save it back to the DB, of course I've made sure that both the first entity and the entity that is going to be inserted are retrieved and fine, it is just thrown upon the save function invokation.
Here's the exception
[err] org.springframework.dao.DataIntegrityViolationException: Attempt to persist detached object "repository.entities.RequestEntity-0". If this is a new instance, make sure any version and/or auto-generated primary key fields are null/default when persisting.; nested exception is <openjpa-2.4.3-r422266:1833086 nonfatal store error> org.apache.openjpa.persistence.EntityExistsException: Attempt to persist detached object "repository.entities.RequestEntity-0". If this is a new instance, make sure any version and/or auto-generated primary key fields are null/default when persisting.
FailedObject: repository.entities.RequestEntity-0
The entity
#Entity
#Table(name="REQUEST")
public class RequestEntity implements Serializable {
private static final long serialVersionUID = 1L;
#Id
#Column(name="REQUEST_ID")
private long requestId;
some other fields ....
//bi-directional many-to-one association to MStatus
#ManyToOne
#JoinColumn(name="STATUS")
private MStatus mStatus;
getters and setters here as well ..
}
And lastly, here's the code
private void doStuff() throws Exception {
List<RequestEntity> requestsList = requestRepo
.findByMStatusStatusContaining("TEXT");
RequestEntity requestItem;
if (requestsList.size() > 1 || requestsList.isEmpty()) {
throw new Exception("No requests found");
} else {
requestItem = requestsList.get(0);
}
requestItem.setMApprovalStatus(mapprovalStatus.findOne("TEXT_TWO"));
requestRepo.save(requestItem);
}