integreat/.claude/skills/ssr-form-migration/reference/form-vs-wizard.md

# Forms vs. wizards (and the data-driven wizard engine)

## Classify first

| Signal | Classification |
|--------|----------------|
| One logical step — even with a `?mode=` toggle, $/% radio, or add/remove rows | **plain form** |
| The user genuinely advances through ordered steps, each validated before the next | **wizard** |
| In doubt | **form** |

Most "wizards" in this codebase are single-step forms wearing wizard costumes: they
implement the multi-step protocol (`mm/ModalWizardStep` + friends), serialize an EDN
snapshot into hidden fields, and register 10–20 stacked-middleware routes — all for one
step. That is pure overhead to delete.

> **Done — Transaction Edit is now a plain form.** `LinksStep`/`EditWizard` and all `mm/*`
> usage were deleted from `transaction/edit.clj`; the worked example below is realized, not
> aspirational. See "Single-step → plain form (realized)".

## The machinery being replaced

The old shape (kept here as the "before"):

```clojure
(defrecord LinksStep [linear-wizard]
  mm/ModalWizardStep
  (step-name   [_] "Transaction Actions")
  (step-key    [_] :links)
  (edit-path   [_ _] [])
  (step-schema [_] (mm/form-schema linear-wizard))
  (render-step [this {{:keys [snapshot step-params]} :multi-form-state :as request}] ...))
```

…plus the snapshot round-trip: the whole accumulating form state is serialized to hidden
fields (custom EDN readers), then rebuilt every request by merging the posted pieces back
into the snapshot (`:multi-form-state :snapshot` is read ~75× in `edit.clj`). The
serialization needs custom readers, the merge is error-prone, and the payload grows each
step.

---

## Single-step → plain form

Two routes: `GET` (render) and `POST` (validate + save). State is plain form fields + an
entity id. No snapshot, no server state, no protocol.

```clojure
{::route/edit        (fn [req] (html-response (render-edit-form {:entity (get-entity req)})))
 ::route/edit-submit (fn [req] (validate-and-save req))}
```

A `?mode=` toggle is just the `GET` re-rendering with a different query param — still a
plain form. An add-row interaction is one extra `POST` that appends a fresh row and
re-renders (the `+1` route).

### Single-step → plain form (realized: Transaction Edit)

What replacing the wizard actually looked like, end to end:

1. **Delete the records + middleware.** `EditWizard`/`LinksStep`, `mm/open-wizard-handler`,
   `mm/next-handler`, `mm/submit-handler`, `mm/wrap-wizard`, `mm/wrap-decode-multi-form-state`,
   and the `edit-wizard-navigate` route all go. `render-step` becomes a plain `render-form`.
2. **Rename the fields off `step-params[...]`.** Field names are now the schema path
   directly (`(path->name2 :transaction/accounts 0 :transaction-account/account)` →
   `transaction/accounts[0][transaction-account/account]`). They decode straight into the
   form schema via the unchanged `wrap-nested-form-params` + `mc/decode` — no two-key
   snapshot/step-params decode. **Strip stray keys after decode** (`select-keys` to the
   schema's keys) or a non-schema input like the tab group's `method` hidden 500s the save
   (see `gotchas.md`).
3. **Flat state.** `wrap-derive-state` builds a plain `{:snapshot :edit-path :step-params}`
   map (not the `MultiStepFormState` record): `entity-only` fields from the entity, editable
   fields from the live posted form (absent = cleared). The ~34 `:snapshot` reads keep
   working.
4. **Validation/error flow without `wrap-ensure-step`.** Reuse the generic
   `wrap-form-4xx-2` directly: `(-> submit-edit (wrap-form-4xx-2 render-form-response) …)`.
   `submit-edit` runs `assert-schema` then dispatches the save; on a throw, `wrap-form-4xx-2`
   re-renders the whole form with `:form-errors` keyed by schema paths. A `*errors*` dynamic
   var (bound by `render-form`) replaces the form-cursor's `*form-errors*` for field lookups.
5. **Routes shrink to** `edit-wizard` (GET open), `edit-submit` (POST), `edit-form-changed`
   (POST whole-form re-render for dependent changes), `location-select` (GET),
   `unlink-payment` (POST).

---

## Genuinely multi-step → data-driven engine with session-stored step state

> **Inspiration — Django `formtools` `WizardView`.** Django's wizard does *not* round-trip
> a serialized blob of the whole form through the page. Each step's validated data is
> written to a **storage backend (the user session by default)** under that step's key,
> and the steps are combined only at the very end via `get_all_cleaned_data()`. We adopt
> the same model: **replace the EDN snapshot + piecewise merging with per-step form state
> stored in the Ring session.** A step writes its own data under its own key; nothing is
> merged into a snapshot and nothing about other steps rides through the form.
> Refs: `formtools.wizard.views.WizardView`, `SessionStorage`, `get_all_cleaned_data()`
> (https://django-formtools.readthedocs.io/en/latest/wizard.html).

A wizard is **data**:

```clojure
(def vendor-wizard-config
  {:steps [{:key :info  :schema info-schema  :fields [...] :render render-info-step
            :next (fn [data] :terms)}
           {:key :terms :schema terms-schema :fields [...] :render render-terms-step
            :next (fn [data] :done)}]
   :init-fn (fn [req] {...})
   :submit-route "/admin/vendor/wizard/submit"
   :done-fn (fn [all-data req] (save! all-data) (html-response "Saved"))})
```

with a tiny engine (no protocols) whose state lives **in the session**, keyed by a wizard
instance id, each step's data under its own step key — the formtools `SessionStorage`
model. No snapshot, no custom EDN readers, no merge-into-snapshot:

```clojure
;; Storage backed by the Ring session. Path: [:wizards <wizard-id> :step-data <step-key>]
(defn create-wizard! [session config]
  (let [id (str (java.util.UUID/randomUUID))]
    [id (assoc-in session [:wizards id]
                  {:current-step (-> config :steps first :key) :step-data {}})]))

(defn put-step [session id k data] (assoc-in session [:wizards id :step-data k] data)) ; replace, not merge
(defn set-step [session id k]      (assoc-in session [:wizards id :current-step] k))
(defn get-all  [session id]        (->> (get-in session [:wizards id :step-data]) vals (apply merge)))
(defn forget   [session id]        (update session :wizards dissoc id))
```

The render emits only a **reference token** (`wizard-id`, `current-step`) in the form —
never the form's state. The submit handler validates the posted step, `put-step`s it,
computes `:next`, and either advances (`set-step`) or finishes (`get-all` + `:done-fn` +
`forget`). Every fn returns the updated session for the handler to thread into the Ring
response (`(assoc resp :session session')`).

**Two routes per wizard:** open (`partial open-wizard config`) and submit
(`partial handle-step-submit config`). State is namespaced by `wizard-id` inside the
session, so multiple in-flight wizards (and browser tabs) don't collide, and it is
discarded on completion (`forget`).

### Storage lifetime (Open decision 1)

State lives in the Ring session, scoped to true multi-step wizards (plain forms hold
none). Lifetime follows the session; `forget` on completion prevents session bloat. For
long-lived wizards, confirm the session backend (in-memory vs. durable) is acceptable or
pick a durable store.

## The engine — REALIZED (Phase 6)

Built and REPL-proven in Phase 6 as two namespaces (no protocols, no defrecords):

- **`auto-ap.ssr.components.wizard-state`** — the pure session-storage layer (the skeleton
  above, fleshed out): `create-wizard!` / `instance` / `exists?` / `current-step` /
  `context` / `step-data` / `put-step` (replace) / `set-step` / `get-all` / `forget`. Each
  is `session -> session'` (or a read); nothing mutates global state. `:context` holds
  read-only data the steps need (e.g. an entity id) **outside** `:step-data`, so it never
  gets merged into the combined result.
- **`auto-ap.ssr.components.wizard2`** — the engine: `open-wizard`, `render-wizard`,
  `handle-step-submit`, and the `wizard-form` shell. A wizard is a **config map**:

  ```clojure
  {:name :vendor  :form-id "wizard-form"  :submit-route "<resolved url>"
   :init-fn (fn [request] {:context {...} :init-data {step-key data}})
   :done-fn (fn [all-data request] ring-response)
   :steps [{:key :info
            :decode   (fn [request] -> data-map)        ; parse this step's posted fields
            :validate (fn [data request] -> errors|nil)  ; optional
            :render   (fn [ctx] -> hiccup)               ; step body; engine wraps the <form>
            :next     (fn [data] -> next-step-key | :done)}
           ...]}
  ```

  The step's `:render` gets `{:wizard-id :current-step :context :all-data :step-data
  :errors :request :config}`. `:all-data` (every step combined so far) is exactly what a
  **read-only summary/preview step** consumes. Nav buttons post a `direction` field:
  `"next"` (validate+advance via `:next`), `"back"` (no validate), `"submit"` (== next, for
  the last step). Only `wizard-id` + `current-step` ride in the form — **no snapshot**.

**Two routes per wizard:** `(partial open-wizard config)` (GET) and
`(partial handle-step-submit config)` (POST). No `wrap-wizard` / `wrap-decode-multi-form-state`
stack — the engine threads the session itself and `(assoc resp :session session')`.

**Proven via REPL** (lifecycle, before any modal used it): open seeds session state and
renders step 1 with no accumulated data in the form; next stores `{step-key data}` and
advances; an invalid step re-renders itself with errors (no advance); the final step's
`:done` calls `done-fn` with the combined `get-all` data and `forget`s the instance; back
navigates without validating; an unknown/expired `wizard-id` re-opens fresh instead of
500-ing. See the lifecycle eval in the Phase 6 commit message.

**Note (Phase 6 fit).** Transaction Rule itself is *edit + read-only preview of one
entity*, not a true multi-data-step flow — so it exercises the engine's render/navigation/
preview path (`:all-data` feeds the test table) but not the cross-step *merge*. The merge
(`get-all` combining independent steps) gets its real workout in Phase 7+ (Invoice Pay,
New Invoice, Vendor, Client), where steps collect genuinely different fields.

## Conditional `:next` + dual-purpose (new+edit) — New Invoice (Phase 8)

A step's `:next` is just `(fn [data] -> next-step-key | :done)`, so **branching the flow is a
one-liner** — no `CustomNext` protocol, no 308-redirect-to-submit hack:

```clojure
{:key :basic-details
 :next (fn [data] (if (= :customize (:customize-accounts data)) :accounts :done))}
```

`:default` skips the expense-accounts step entirely (the done-fn uses the vendor's default
account); `:customize` routes through the grid. The old wizard expressed this with
`mm/CustomNext` returning either `navigate-handler{:to :accounts}` or a 308 to the submit
route — and the 308 path was broken (see `gotchas.md`, the `{}`→nil 500). The engine's
conditional `:next` is both simpler and correct.

**Dual-purpose (create *and* edit) = one config, one `:init-fn` that branches on a route id:**

```clojure
(defn new-init-fn [request]
  (if-let [id (->db-id (get-in request [:route-params :db/id]))]
    {:init-data {:basic-details (… entity prefilled, :customize-accounts :customize)
                 :accounts {:invoice/expense-accounts (… existing rows)}}}   ; edit
    {:init-data {:basic-details {:invoice/date (coerce/to-date (time/now))   ; new
                                 :customize-accounts :default}}}))
```

`create-wizard!` stores `:init-data` **as the per-step `:step-data` map directly**, so seeding
`{:basic-details … :accounts …}` opens both steps populated — the edit case repopulates the
grid without a separate hydrate. Two open routes (`new-wizard`, `edit-wizard`) both reduce to
`(partial wizard2/open-wizard config)`; the done-fn branches on `(:db/id all-data)` to return
the next-steps modal (create) vs the swapped table row (edit).

**Async step fragments read the posted form, not multi-form-state.** The basic-details
fragments (account-prediction radio, due-date / scheduled-payment suggestions) and the
accounts totals all post the whole `#wizard-form`; in the engine that form carries the flat
`invoice/*` fields + the opaque `wizard-id`, so a fragment decodes what it needs straight from
`form-params` (and, for a cross-step value like the invoice total on the accounts step, reads
`ws/get-all` via the posted `wizard-id`). No `mm/wrap-decode-multi-form-state` stack survives.