It's cool to see people working on library relations, have you seen https://github.com/BoxyUwU/librelations its out of date but relies on a fork of bevy_ecs to allow for despawning entities to also despawn the targets of their relations. I don't do anything with bevy anymore these days, best of luck with your relations impl and hope my previous impl might be interesting to you

The derive macro currently doesn't support generic ZSTs, leaving me to do this instead:

pub struct Multiply<C: Composite>(PhantomData<fn(C)>);

impl<C: Composite> Relation for Multiply<C> {
   const CLEANUP_POLICY: CleanupPolicy = CleanupPolicy::Orphan;
   const EXCLUSIVE: bool = true;
   const SYMMETRIC: bool = false;
}

At a glance aery only uses the bevy_ecs and bevy_derive crates, would you be open to restricting the bevy dependency to these? It could reduce compile times and make aery more feasable for non-graphics applications.

Rather than filtering by entities who are part of a relation, I want to filter by entities who are not involved in the relation.

Effectively, this:

#[derive(WorldQuery)]
pub struct Abstains<R: Relation> {
    // is not a host nor a target of relation R
    filter: (Without<Participant<R>>, Without<RootMarker<R>>),
}

Looking for these methods implemented on Option<&RelationsItem<'_, RS>> as well:

I've run into this need a decent bit in my current project, and can't seem to implement iter_hosts/iter_targets as an extension myself since EdgeIter is currently private.