How to use "apply_system_buffers" or schedule commands properly

[rowanfr]

I'm just trying to come to understand how to interact with the scheduling systems in bevy. I've written a simple script in bevy 0.10.1 which first uses the Commands struct to queue to spawn an entity with a num(u32) component. I then chain together "apply_system_buffers" followed by a Query for that num component for which I deliberately use .get_single().unwrap().0 to cause an error if the entity didn't spawn before the function call. The full code is below:

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems((command_spawn,apply_system_buffers,query).chain())
        .run();
}

#[derive(Component)]
struct num(u32);

fn command_spawn(
    mut commands: Commands,
) {
    commands.spawn(num(12));
}

fn query(
    query: Query<&num>
){
    println!("{}",query.get_single().unwrap().0);
}

If I use ".add_startup_system(command_spawn)" it prints out the expected value 12 repeatedly but using .chain() with apply_system_buffers buffer or using

.add_system(command_spawn.in_base_set(StartupSet::Startup)) 
.add_system(query.in_base_set(CoreSet::Update))

in place of .add_systems((command_spawn,apply_system_buffers,query).chain()) both result in the program crashing with it panicking at the unwrap() as expected indicating that the Commands weren't resolved before the execution of the next system.

In summary I'm just trying to figure out what I did wrong and how can I organize the scheduling of systems to follow my intentions.

[rowanfr]

Ah I know what I did wrong. Obviously don't add a system to an app which repeatedly spawns something while also trying get a single, additionally don't code at 3 in the morning.

When implementing everything properly everything works as expected

For the full proper implementation use:
.add_startup_systems((command_spawn,apply_system_buffers,query).chain())
This will work as expected and print out the value 12 once. Additionally removing apply_system_buffers while still keeping them in a chain can sometimes still allow it to function (but obviously don't rely on this) while removing the chain guarantees that an error will be thrown.

[debone]

For future reference, apply_system_buffer has been renamed to apply_deferred since Bevy 0.11