How can i use if condition in “create_query” in symfony2

Question

In my form builder i am trying to use this

'query_builder' => function(EntityRepository $er) {
                                 return $er->createQueryBuilder('u')
                                 if(var)                                     
                                  ->where('u.id = 3')

Answer 1

'query_builder' => function(EntityRepository $er) {
    $qb = $er->createQueryBuilder('u');

    if ($someCondition) {
        $qb->where('u.id = 3');
    }

    return $qb;
}