SEVERE: Error while trying to create a child.
javax.persistence.RollbackException: Exception [EclipseLink-4002] (Eclipse Persistence Services - 2.7.2.v20180622-f627448): org.eclipse.persistence.exceptions.DatabaseException
Internal Exception: org.postgresql.util.PSQLException: ERROR: null value in column "parent_id" violates not-null constraint

public void add(Tenant tenant, Parent parent) {
    runInTransaction(tenant, entityManager -> {
        final List<Child> children = parent.getChildren();
        parent.setChildren(Collections.emptyList());
        entityManager.persist(parent);
        children.forEach(child -> {
            child.setParent(parent);
            entityManager.persist(child);
        });
    });
}

protected void runInTransaction(Tenant tenant, Consumer<EntityManager> action) {
    final EntityManager manager = getEntityManager(tenant);
    manager.getTransaction().begin();
    manager.createNativeQuery(String.format("SET LOCAL search_path TO %s, public;", tenant.getSchemaName())).executeUpdate();
    action.accept(manager);
    manager.getTransaction().commit();
}