Recipe: forms, validation errors, precognition

When: any form that writes. Rails: form_with + if @model.save … else render :new (in Inertia Rails: redirect_to …, inertia: { errors: }).

The loop

1. The page renders Inertia's <Form action={routes.update(id)}>; every input has a name.
2. The handler parses with Params<T> (query + JSON or form body, unknown keys ignored).
3. Validate on the model. Success: Redirect::to(path).notice("…"). Failure: Redirect::to(form_path).errors(errors) or Redirect::back(&headers, fallback).errors(errors).
4. The errors ride the flash cookie to the next GET and arrive as errors ({field: string[]}), which <Form> exposes: errors.name?.map((message) => ({ message })) into shadcn's <FieldError>.

Working examples: src/controllers/settings/profiles.rs + frontend/pages/settings/profiles/show.tsx (hand-written), or any scaffolded resource (src/controllers/<plural>.rs, form.tsx).

Model side

use crate::models::users::{Errors, SaveError};

let mut errors = Errors::new();
if name.trim().is_empty() { errors.add("name", "can't be blank"); }
if !errors.is_empty() { return Err(SaveError::Invalid(errors)); }

• Messages match Rails' defaults: "can't be blank", "is not a number", "has already been taken", "is too short (minimum is 12 characters)". Check the Rails kit when porting one.
• Forms submit strings. src/models/cast.rs casts them (cast::string, number, optional_date, boolean) and adds those messages; #[serde(deserialize_with = "cast::form_value")] also accepts JSON numbers/booleans from API clients.
• A field that must tell "missing" from null: controllers::nullable (see settings/profiles.rs).

Precognition (validate as you type)

Server: before writing, answer a Precognition: true request with the errors and return:

if let Some(res) = precognitive(&headers, &params.errors()) {
    return Ok(res); // 422 {errors} or 204 + Precognition-Success; nothing written
}

params.errors() must not touch the database for writes, send mail, or spend a rate-limit token. Endpoints that can't validate without side effects take _: NoPrecognition as their first extractor, which answers 400. Scaffolded create/update already do the former and destroy the latter.

Client: add validate calls to the fields:

<Form action={projects.create()}>
  {({ errors, validate }) => (
    <Input name="name" onBlur={() => validate("name")} />
  )}
</Form>

Error bags, several forms on one page

Give each <Form> an errorBag="delete_user": the client sends it as X-Inertia-Error-Bag, and the adapter nests that request's errors under the key, so two forms don't show each other's errors. Redirect::to(..).errors(e).error_bag("delete_user") sets it from the server instead. None of the kit's pages needs one yet; the behaviour is covered in tests/inertia_b.rs.

Verify

let res = server.patch(path).json(&json!({ "name": "" })).await;
assert_redirect(&res, path);
let page = inertia_get(&server, &ctx, path).await;
assert_eq!(page["props"]["errors"], json!({ "name": ["can't be blank"] }));

Precognition: tests/requests/precognition.rs shows the pattern (asserts the database is unchanged, 422 with errors, 204 when valid).