render stages: Can't use an entity spawned in the same stage

[dcz-self]

Bevy version

0.9.1

What you did

The use case: I want to have a uniform which is created based on 2 separate sets of extracted data. I'm bound on one side by the Extract stage where the extraction takes place, and the Prepare stage, where the instantiated uniform is written to the buffer via UniformComponentPlugin.

There is no stage between them (adding a stage results in the system not running).

Ordering systems within one stage doesn't let the later systems see the result of the earlier. See the code below:

use bevy::prelude::*;
use bevy::render::{RenderApp, RenderStage};

fn main() {
    let mut app = App::new();
    if let Ok(render_app) = app.get_sub_app_mut(RenderApp) {
        render_app
            .add_system_to_stage(RenderStage::Prepare, spawner_system)
            .add_system_to_stage(RenderStage::Prepare, looker.after(spawner_system));
    }
    app.run();
}
#[derive(Component, Debug)]
struct MyUniform;

fn spawner_system(mut commands: Commands) {
    commands.spawn(MyUniform);
}

fn looker(query: Query<(Entity, &MyUniform)>) {
    for (e, u) in query.iter() {
        dbg!(e, u);
    }
}

What went wrong

Expected: something is printed, i.e. spawned etities are available in the next stage which is explicitly serialized.

Actual: nothing is printed. Entities available in the next stage (not illustrated here).

[dcz-self]

I was educated that commands take effect between stages, making this invalid.

[hymm]

Fyi your minimal example is a little too minimal. The render sub app is added by the Render plugin and that isn't getting added here.