hibernate - In JPA , Is em.flush() is exceptional usage or not -

hi i'm updating parent entity (department) has child(employees) ,along update want delete child , add new set of child . , have unique constraint employee_name db side . first i'm finding department , calling clear (collection clear ) list of employees adding new child department , committing . i'm getting unique constraint violation .if use em.flush() after clear() deleting child , inserting new child . know flush() exceptional use, commit uses flush() internally . there solution delete , insert child in single transaction ?? , i'm using orphanremoval = true in department entity

public void deleteemployee(department updatingdepartment){     list<employee> employees = new arraylist<employee>();     employee employee1 = new employee();     employee1.setemployeealary(16667);     employee1.setemployeename("manju");       employee employee2 = new employee();     employee2.setemployeealary(16667);     employee2.setemployeename("sunil");        entitymanager.gettransaction().begin();      department founddepartment = entitymanager.find(department.class, updatingdepartment.getdepartmentid());      founddepartment.getemployees().clear();           employee2.setdepartment(founddepartment);         employee1.setdepartment(founddepartment);         employees.add(employee1);         employees.add(employee2);            /*entitymanager.flush();*/         founddepartment.getemployees().add(employee1);         founddepartment.getemployees().add(employee2);          entitymanager.merge(founddepartment);      entitymanager.gettransaction().commit();  } 

first of don't need merge entity still in attached state (retrieved fron entitymanager not disposed yet). see entity life cycle.

you need understand what's merge function jpa entitymanager: why use persist() on merge()?

in fact here have nothing except commiting , closing em, changes automatically reported.

then, orphanremoval feature 1 use in case (remove child entities when removed parent collection) should trick.

concerning 'flush() / clear()' right use case, it's when you're processing batch operation on lot's of entities , don't want them stay in level 1 cache (entitymanager one) in order limit memory consumption.

you can have : struggling understand entitymanager proper use