Relationship API is unfriendly to Entity-like newtypes

[Zeenobit]

What problem does this solve or what need does it fill?

Earlier I opened a similar issue regarding events: #21384
With PR 21408 attempting to fix the issue for entity events, I want to apply similar changes to the relationship API.

Current problems:

1. The current implementation of derive_relationship again makes hard assumptions about the underlying field being exactly an entity and nothing else. Similar to EntityEvent, Relationship::get this should work with an ContainsEntity type.
2. Similar to mutable vs. immutable events discussed in the PR, relationships should be immutable by default. As far as I can tell, the mutability is only required for the "fancy" relationships constructors like EntityCommands::with_related or RelatedSpawner. I see no downside to disallow these functions in exchange for ensuring the relationship component is immutable.
3. As is, RelationshipSourceCollection cannot be implemented for containers of entitoids (i.e. Vec<Instance<T>>).
   • This is a much harder problem to solve. There is just no way to block collection mutation and keep relationships functional. I'm ok living with this, but I just wanted to raise the issue in case anyone has ideas.

What solution would you like?

I would like to solve at least problems 1 and 2.

1. derive_relationship can easily use ContainsEntity::entity(self.#relationship_field) to support entitoids
2. We could break out from and set_risky from Relationship into a SetRelationship trait and require it for the fancier setup functions. We could then derive this trait only if #[relationship(mutable)] is specified (or maybe if #[component(immutable)] is NOT specified)

What alternative(s) have you considered?

N/A

[alice-i-cecile]

I'm also in favor of this and #1634.

[Freyja-moth]

As far as I can tell, the mutability is only required for the "fancy" relationships constructors like EntityCommands::with_related or RelatedSpawner

I don't believe either of these rely on relationship mutability, since they both just insert a relationship onto the entity.

Or are you talking about making RelationshipTarget immutable?

[Zeenobit]

@Freyja-moth No, I'm specifically talking about Relationship::from and Relationship::set_risky methods. There is no reason for Bevy to HAVE to mutate Relationship components. The user can construct/insert Relationship components, Bevy can manage RelationshipTarget.