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"):

(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.

{::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:

(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:

;; 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-steps 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:

  {: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 forgets 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.