Query Filters cause additional tuple layer with () as the 2nd entry - is this expected?

[bbarker]

  world
      .query::<(Entity, &Component1)>()
      .iter(&world)
      .for_each(|(ent, _)| {
          println!("C1 entitiy: {:?}", ent);
      });
  world
      .query::<((Entity, &Component2), With<SerializeMe>)>()
      .iter(&world)
      .for_each(|(ent, _)| {
          println!("C2 entitiy: {:?}", ent);
      });

This gives output like:

C1 entitiy: 0v0
C1 entitiy: 1v0
C2 entitiy: (1v0, Component2 { target: 0v0 })

I'm just curious if I'm doing these queries in the ideal fashion. They work, but in the second case, ent actually contains the component as part of an inner tuple - the extra With parameter seems to cause () to always be returned. That's fine if it is expected, but doesn't seem to match the examples I've seen.

[rparrett]

Query has two generic types: Query<Q, F>.

In the case of the second example, you're using a big ol' tuple for the Q "query fetch" type, and the default value () for the F "filter type." Effectively Query<((Entity, &Component2), With<SerializeMe>), ()>. You're ending up with a second layer of tuple because you're asking for it, and Query is flexible enough to provide it.

What you want is: (edit: also, use query_filtered)

world
    .query_filtered::<(Entity, &Component2), With<SerializeMe>>()
    .iter(&world)
    .for_each(|(ent, _)| {
        println!("C2 entitiy: {:?}", ent);
    });

Note:

Rust is also flexible enough to destructure this nested tuple:

.for_each(|((ent, _), _)| {

Note 2:

This confusing situation should be resolved by #9918 which would not allow filters in the fetch type.

[bbarker]

Ah ok. I actually had tried the query you specified, but get this error (using 0.12.0 if that matters):

error[E0107]: method takes 1 generic argument but 2 generic arguments were supplied
   --> src/lib.rs:254:14
    |
254 |             .query::<(Entity, &Component2), With<SerializeMe>>()
    |              ^^^^^                          ----------------- help: remove this generic argument
    |              |
    |              expected 1 generic argument
    |

[bbarker]

The issue being that world.query takes a single parameter, of type WorldQuery

[alice-i-cecile]

You need world.query_filtered here :)

[rparrett]

Ah sorry, Alice is correct. I don't use World often and didn't spot that!

I edited the answer above so nobody else trips on it.