bevyengine/bevy · error

{derive} derive expected unnamed structs with one field or w

Error message

{derive} derive expected unnamed structs with one field or with a field annotated with #[relationship].

What it means

Compile error from relationship_field: the derive was applied to a tuple (unnamed) struct with multiple fields and none is marked #[relationship], so the helper cannot pick the relationship field. This branch mirrors the named-struct guard for Fields::Unnamed.

Source

Thrown at crates/bevy_ecs/macro_logic/src/component.rs:817

    match fields {
        Fields::Named(fields) if fields.named.len() == 1 => Ok(fields.named.first().unwrap()),
        Fields::Named(fields) => fields.named.iter().find(|field| {
            field
                .attrs
                .iter()
                .any(|attr| attr.path().is_ident(RELATIONSHIP))
        }).ok_or(syn::Error::new(
            span,
            format!("{derive} derive expected named structs with a single field or with a field annotated with #[relationship].")
        )),
        Fields::Unnamed(fields) if fields.unnamed.len() == 1 => Ok(fields.unnamed.first().unwrap()),
        Fields::Unnamed(fields) => fields.unnamed.iter().find(|field| {
                field
                    .attrs
                    .iter()
                    .any(|attr| attr.path().is_ident(RELATIONSHIP))
            })
            .ok_or(syn::Error::new(
                span,
                format!("{derive} derive expected unnamed structs with one field or with a field annotated with #[relationship]."),
            )),
        Fields::Unit => Err(syn::Error::new(
            span,
            format!("{derive} derive expected named or unnamed struct, found unit struct."),
        )),
    }
}

View on GitHub (pinned to 396ca72708)

Solutions

1. Mark the intended field with #[relationship]: struct X(#[relationship] Entity, Foo);
2. Shrink the tuple struct to a single field such as struct ChildOf(Entity);
3. Use a named struct with one field instead

When it happens

Trigger: Thrown at crates/bevy_ecs/macro_logic/src/component.rs:817 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of bevyengine/bevy@396ca72708 (2026-08-20). Data as JSON: /api/errors/2edb926e84993929. Report an issue: GitHub.