Accessing multiple entities in QueryData/WorldQuery impl

[BigWingBeat]

I have two component types, Foo and Bar. Foo holds an Entity value which always points to an entity with a Bar component, and this is a many-to-one relationship, so multiple Foo entities can exist that all point to the same Bar entity. The catch is that in order to do anything useful with a Foo, you also need a &mut reference to its corresponding Bar. As this is for a library, I want to hide this relationship as an implementation detail, so that users can simply query for Foo, and the library handles retrieving the corresponding Bar internally.

There are three ways I can think of to achieve this:

1. Don't bother, just push all the bookkeeping onto the user. Terrible UX.
2. Create a custom SystemParam type. Would require a lot of boilerplate manually duplicating Bevy's Query API.
3. Create a custom QueryData/WorldQuery type. I would prefer this, but I don't understand the ECS internals enough to determine if would be sound for the impl to fetch both the Foo value for the requested entity, as well as the Bar value from whatever entity is specified by the Foo.

Is it possible to soundly do 3, or is there a better solution?

[BigWingBeat]

(Cross-posted from Discord: https://discord.com/channels/691052431525675048/1253399316773736520)

[BigWingBeat]

Answer from Giuschi#7708 on Discord:

I don't think this is possible to do with a query. Query assumes that you can get all its elements at once (e.g. with query.iter_mut().collect()). However if in your example you would yield a &mut Foo multiple times (one for each Bar connected to it), which is not sound according to Rust's rules.
You'll have to separately query for &mut Foos and borrow from that query when you want to do something with a Bar. You could possibly wrap this in a custom SystemParam to make it a bit more ergonomic, but fundamentally you'll have to do the query in separate steps.
I don't think relationships will help because you likely won't be able to modify the relationship while iterating (only through commands for example), meaning no &mut Foo access.

TL;DR No it isn't sound, and 2 is the most reasonable solution.