java - Adding a newly persisted entity to a list of entities held by the inverse side of a relationship so as to maintaining a bidirectional relationship -

given 2 entities department , employee forming one-to-many relationship department employee.

_{since relationship quite intuitive, leaving out entity classes.}

the following segment of code, persists entity employee.

public void insert() {     employee employee = new employee();     employee.setemployeename("k");      department department = entitymanager.find(department.class, 1l);     employee.setdepartment(department);     entitymanager.persist(employee);     entitymanager.flush();      list<employee> employeelist = department.getemployeelist();     employeelist.add(employee); } 

and following method returns list of employees associated particular department.

public list<employee> getlist() {     return entitymanager.find(department.class, 1l).getemployeelist(); } 

both methods written in stateless ejb using cmt (hereby not bmt) named let's employeeservice.

a client application invokes these methods in sequence so,

employeeservice.insert(); list<employee> employeelist = employeeservice.getlist();  (employee e : employeelist) {     system.out.println(e.getemployeeid() + " : " + e.getemployeename()); } 

the sout statement in foreach loop above displays newly added employee entity list<employee> in department null employeeid in given line entitymanager.flush(); not present in first code snippet.

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entitymanager#persist(object entity) not guaranteed generate id. id guaranteed generated @ flush time.

what happens is, if entitymanager.flush(); removed/commented, entity employee added list of employees (list<employee> employeelist) null identifier in (the primary key column in underlying database table).

what usual way maintain bidirectional relationship? entitymanager#flush(); needed every time entity added collection of entities being maintained inverse side of relationship generate id associated newly persisted entity?

_{also, required manually delete employee list<employee> (maintained inverse side of relationship - department) while deleting employee entity (using entitymanager.remove(employee);)?}

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edit : entity classes :

department :

@entity @table(catalog = "testdb", schema = "", uniqueconstraints = { @uniqueconstraint(columnnames = {"department_id"})}) public class department implements serializable {      @id     @generatedvalue(strategy = generationtype.identity)     @basic(optional = false)     @column(name = "department_id", nullable = false)     private long departmentid;      @column(name = "department_name", length = 255)     private string departmentname;      @column(length = 255)     private string location;     @onetomany(mappedby = "department", fetch = fetchtype.lazy)     private list<employee> employeelist = new arraylist<employee>(0);      private static final long serialversionuid = 1l;     // constructors + getters + setters + hashcode() + equals() + tostring(). } 

employee :

@entity @table(catalog = "testdb", schema = "", uniqueconstraints = {     @uniqueconstraint(columnnames = {"employee_id"})}) public class employee implements serializable {      @id     @generatedvalue(strategy = generationtype.identity)     @basic(optional = false)     @column(name = "employee_id", nullable = false)     private long employeeid;      @column(name = "employee_name", length = 255)     private string employeename;     @joincolumn(name = "department_id", referencedcolumnname = "department_id")     @manytoone(fetch = fetchtype.lazy)     private department department;      private static final long serialversionuid = 1l;      // constructors + getters + setters + hashcode() + equals() + tostring(). } 

when persisting employee, need set both sides of association.

in department should have method:

public void addemployee(employee employee) {     employees.add(employee);     employee.setdepartment(this); } 

make sure cascade de persist , merge events children association:

@onetomany(cascade = cascadetype.all, mappedby = "department", orphanremoval = true) private list<employee> children = new arraylist<>(); 

and persisting logic becomes:

employee employee = new employee(); employee.setemployeename("k");  department department = entitymanager.find(department.class, 1l); department.addemployee(employee);