Custom 1:1 relationship not inserted when using `.with_related()`

[TravisRybka]

Bevy version and features

• 0.18.0 (repro'd on main at cf35f37)

What you did

Created a custom 1:1 relationship, and spawned entities using EntityCommands::with_related:

#[derive(Component)]
#[relationship(relationship_target = MyFriend)]
struct FriendOf(pub Entity);

#[derive(Component)]
#[relationship_target(relationship = FriendOf)]
struct MyFriend(Entity);

Spawning entities:

fn spawning(mut commands: Commands) {
    let mut top_entity_command = commands.spawn_empty();

    // Bugged Behavior
    top_entity_command.with_related::<FriendOf>(Bugged);

    // Workaround
    let top_entity_id = top_entity_command.id();
    commands.spawn((FriendOf(top_entity_id), NotBugged));
}

// Marker Trait
#[derive(Component)]
struct Bugged;

// Marker Trait
#[derive(Component)]
struct NotBugged;

What went wrong

Using .with_related::<FriendOf>() doesn't insert the FriendOf component on the source entity.

fn verify(
    bugged: Single<Has<FriendOf>, With<Bugged>>,
    not_bugged: Single<Has<FriendOf>, With<NotBugged>>,
) {
    // Prints "false"
    println!("Bugged:\t\t{}", bugged.into_inner());
    // Prints "true"
    println!("Not bugged:\t{}", not_bugged.into_inner());
}

Additional information

This seems to be specific to 1:1 relationships (i.e. impl RelationshipSourceCollection for Entity). If I change the relationship target to be a Vec<Entity>, there is no bug. E.g.

-- struct MyFriend(Entity);
++ struct MyFriend(Vec<Entity>);

With the above diff, then the verify system returns "true" and "true"