Spawning related entities in an OnAdd observer downstream of a World::spawn in a Command causes a crash

[Jaso333]

Bevy version

main

What you did

Created an OnAdd observer for a component that adds related (ChildOf) entities using Commands. Then, spawn an entity with that component in an implementation of Command::apply using World::spawn. Here is the MRE (excluding App creation and observer/system insertion):

#[derive(Component)]
struct MyComponent;

fn on_add(trigger: Trigger<OnAdd, MyComponent>, mut commands: Commands) {
    commands
        .entity(trigger.target())
        .with_related::<ChildOf>(|rsc| {
            rsc.spawn_empty();
        });
}

struct MyCommand;

impl Command for MyCommand {
    fn apply(self, world: &mut World) -> () {
        world.spawn(MyComponent);
    }
}

fn startup(mut commands: Commands) {
    commands.queue(MyCommand);
}

What went wrong

It is claimed that the entity doesn't exist. The following error is emitted, and the app crashes:

thread 'main' panicked at C:\Users\Jason\.cargo\git\checkouts\bevy-50d7e162b728c6c6\821f6fa\crates\bevy_ecs\src\error\handler.rs:129:1:
Encountered an error in command `<bevy_ecs::system::commands::entity_command::insert<bevy_ecs::hierarchy::ChildOf>::{{closure}} as bevy_ecs::error::command_handling::CommandWithEntity<core::result::Result<(), bevy_ecs::world::error::EntityMutableFetchError>>>::with_entity::{{closure}}`: EntityDoesNotExist(EntityDoesNotExistError { entity: 11v1#4294967307, details: EntityDoesNotExistDetails { location: MaybeLocation { marker: PhantomData<core::option::Option<&core::panic::location::Location>> } } })

Additional information

The same thing happens with on_add hooks directly on the component as opposed to an observer. If I spawn the component using Commands instead of World, it works as expected.

In version 0.15.x, I was able to perform spawning using this exact pattern without this error.

[Jaso333]

To add to this: if I change world.spawn(MyComponent) to world.commands().spawn(MyComponent), it works.

[ElliottjPierce]

I'll start investigating this since I don't think anybody else is. Stop me if I'm stepping on your toes.