Constant Fields

A constant is an output-only field whose value is computed once, at creation, and never accepted from input or changed by an update (e.g. id).

• Declared with #[constant(value_or_resolver)] -- a static value, or a |ctx, opts| -> T resolver (sync or async, with access to context and options like any other handler).
• Requires output(...) on the schema, since it never appears on the input struct.
• May have on_delete and on_success event handlers.

Example

id is a static constant. label is computed once via a zero-argument resolver closure:

use ivo::ivo_schema;

#[ivo_schema(
    input(ItemInput, derive(Debug, Clone, PartialEq)),
    output(Item, derive(Debug, Clone, PartialEq))
)]
mod item_schema {
    struct Fields {
        #[constant(1234)]
        pub id: i32,

        #[constant(|| "generated".to_string())]
        pub label: String,

        #[required]
        #[validate(|v: String, _, _| Ok(Some(v)))]
        pub name: String,
    }
}

fn main() {
    let (created, _ctx_options) = item_schema::ItemModel
        .create(
            item_schema::PartialItemInput {
                name: Some("widget".into()),
            },
            (),
        )
        .unwrap();

    assert_eq!(created.id, 1234);
    assert_eq!(created.label, "generated");

    println!("{:#?}", created);
    // Item { id: 1234, label: "generated", name: "widget" }
}

An update to item_schema::PartialItemInput has no id/label fields at all -- there's nothing to submit for a constant, and no way to change it after creation.

Try it in the browser

id is a constant (always 1234); username is lax with a default. Edit the input and run it.

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