liveview-patterns

LiveView Patterns Reference

Ash projects: Use ash-framework skill for AshPhoenix.Form. Lifecycle: AshPhoenix.Form.validate/3 on phx-change, AshPhoenix.Form.submit/2 on submit, to_form/1 for HEEx. Do not use Ecto.Changeset.cast/3.

Reference for building with Phoenix LiveView 1.0/1.1.

Iron Laws — Never Violate These

1. NO UNCONDITIONAL DB QUERIES IN MOUNT — Mount runs TWICE. Default: assign_async. SEO routes: connected? guard + cache-backed disconnected branch (crawlers read that HTML)
2. ALWAYS USE STREAMS FOR LISTS — Regular assigns = O(n) memory per user. Streams = O(1)
3. CHECK connected?/1 BEFORE SUBSCRIPTIONS — Prevents double subscriptions
4. EXTRACT VARIABLES BEFORE assign_async CLOSURE — Closures copy entire referenced variables
5. LOAD PRIMARY DATA IN mount/3, PAGINATION IN handle_params/3 — handle_params runs on EVERY URL change
6. NEVER PASS SOCKET TO BUSINESS LOGIC — Extract data before calling contexts
7. CHECK CHANGESET ERRORS BEFORE UI DEBUGGING — Silent form save = check {:error, changeset} first, not viewport/JS
8. HIDDEN INPUTS FOR ALL REQUIRED EMBEDDED FIELDS — Every required field in an embedded schema MUST have a hidden_input if not directly editable
9. NEVER USE assign_new FOR LIFECYCLE VALUES — assign_new skips the function if key exists. Use assign/3 for locale, current user, or any value refreshed every mount
10. MATCH {:error, %Ecto.Changeset{}} EXPLICITLY — Bare {:error, _} merges changeset and non-changeset errors; the form silently never re-renders validation errors. Handle other errors separately

Memory Impact

Pattern	3K items	10K users × 10K items
Regular assigns	~5.1 MB	~10+ GB
Streams	~1.1 MB	Minimal (O(1))

Decision: Lists with >100 items → Use streams, not assigns

Quick Patterns

Async Assigns (CRITICAL)

def mount(%{"slug" => slug}, _session, socket) do
  # Extract needed values BEFORE the closure
  scope = socket.assigns.current_scope

  {:ok,
   socket
   |> assign_async(:org, fn -> {:ok, %{org: fetch_org(scope, slug)}} end)}
end