#[test]
fn query() {
    struct ChildOf;

    let mut world = World::new();

    let parent1 = world.spawn().id();
    let child1 = world.spawn().insert_relation(ChildOf, parent1).id();
    let parent2 = world.spawn().id();
    let child2 = world.spawn().insert_relation(ChildOf, parent2).id();

    let mut query = world.query::<(Entity, &Relation<ChildOf>)>();
    let mut iter = query.iter_mut(&mut world);
    assert!(iter.next() == Some((child1, ())));
    assert!(iter.next() == Some((child2, ())));
    assert!(iter.next() == None);

    query.set_relation_filter(
        &world,
        QueryRelationFilter::new().add_target_filter::<ChildOf, _>(parent1),
    );
    let mut iter = query.iter_mut(&mut world);
    assert!(iter.next() == Some((child1, ())));
    assert!(iter.next() == None);

    query.set_relation_filter(
        &world,
        QueryRelationFilter::new().add_target_filter::<ChildOf, _>(parent2),
    );
    let mut iter = query.iter_mut(&mut world);
    assert!(iter.next() == Some((child2, ())));
    assert!(iter.next() == None);
}