Querying hierarchies?

[Metadorius]

What's the most optimized way to query for a certain hierarchy of items? I am implementing an RTS camera that I need to orbit around a pivot which will be moving. I skimmed through the docs and seen only a possibility to query for entities with children/a parent, without specifying what components does the parent/the children have.

[rparrett]

One thing that is possible now in Bevy 0.8 is iterating over Children with iter_many.

fn children(query: Query<&Children>, text_query: Query<&Text>) {
    for children in query.iter() {
        for child in text_query.iter_many(children) {
            info!("{}", child.sections[0].value);
        }
    }
}

But other than that, we pretty much have to do a lot of manual work with multiple queries, e.g.

fn parent(query: Query<&Parent>, text_query: Query<&Text>) {
    for parent in query.iter() {
        for text in text_query.get(parent.get()) {
            info!("{}", text.sections[0].value);
        }
    }
}

It's also possible to construct recursive queries like this: #838 (comment) (updating visibility hierarchies this way is thankfully no longer necessary with Bevy 0.8)

There's ongoing discussion about entity-entity relations here:
#3742