@ManyToOne
@JoinColumn(name = "ik_parent_department_id")
private Department parentDepartment;

@OneToMany(mappedBy = "parentDepartment")
private Set children = new HashSet(0);

Department parentDepartment = (Department) session.load(Department.class, id);
session.delete(parentDepartment);
for (Department child : parentDepartment.getChildren()){
    child.setParentDepartment(null);
}
session.flush();

for (Department child : parent.getChildren()) {
    child.setParentDepartment(null);
}
session.delete(parent);

@OneToMany(mappedBy="parent", cascade={CascadeType.PERSIST})
Collection<Child> children;

@PreRemove
private void preRemove() {
   children.forEach( child -> child.setParentnull));
}