music_library/backlog/tasks/ml-169.10 - Fix-Missing-optimistic-UI-for-delete-operations.md

id, title, status, assignee, created_date, updated_date, labels, dependencies, references, modified_files, parent_task_id, priority, ordinal

id	title	status	assignee	created_date	updated_date	labels	dependencies	references	modified_files	parent_task_id	priority	ordinal
ML-169.10	Fix: Async optimistic UI for index/list delete streams	To Do		2026-05-19 11:48	2026-05-20 10:12	perf fix ux		backlog/documents/doc-27 - Phase-4-Performance-Audit-Low-Hanging-Fruit-Findings.md	lib/music_library_web/live_helpers/index_actions.ex lib/music_library_web/live/collection_live/index.ex lib/music_library_web/live/wishlist_live/index.ex lib/music_library_web/live/scrobbled_tracks_live/index.ex test/music_library_web/live/collection_live/index_test.exs test/music_library_web/live/wishlist_live/index_test.exs test/music_library_web/live/scrobbled_tracks_live/index_test.exs	ML-169	medium	31000

Description

Add optimistic UI for delete operations only where the item is rendered from a LiveView stream on an index/list page. The item should be removed from the stream immediately, while the database delete runs through LiveView-owned async handling so failures can be reported and rolled back.

Problem

Index/list delete handlers currently wait for the database delete before updating the UI. stream_delete is used after the delete succeeds, so the interaction can feel slower than necessary. Additionally, both callers use {:ok, _} = pattern match on Records.delete_record/1 and ListeningStats.delete_track/1 — if either returns {:error, changeset}, the LiveView process crashes.

Note: delete buttons already use JS.hide(@hide_target) before JS.push("delete") (RecordComponents line 708, scrobbled tracks line 145). This provides a client-side optimistic hide with no rollback — if the server delete fails, the row stays hidden despite the record still existing in DB. The server-side stream changes in this task provide the rollback mechanism.

This task is intentionally scoped to stream-backed list views only:

• Collection index and wishlist index record streams via IndexActions.handle_delete/2
• Scrobbled tracks index stream

Show-page deletes are out of scope. CollectionLive.Show and WishlistLive.Show navigate away after delete and do not have a list stream to optimistically update. Their failure and rollback behavior should remain synchronous unless a separate navigation-specific design is created.

Files

• lib/music_library_web/live_helpers/index_actions.ex — remove record from :records immediately and start a LiveView async delete operation
• lib/music_library_web/live/collection_live/index.ex — handle async record delete success/failure for collection index
• lib/music_library_web/live/wishlist_live/index.ex — handle async record delete success/failure for wishlist index
• lib/music_library_web/live/scrobbled_tracks_live/index.ex — remove track from :tracks immediately and handle async track delete success/failure
• test/music_library_web/live/collection_live/index_test.exs — add delete tests (currently zero coverage)
• test/music_library_web/live/wishlist_live/index_test.exs — add delete tests (currently zero coverage)
• test/music_library_web/live/scrobbled_tracks_live/index_test.exs — add delete tests (currently zero coverage)

Fix

Use LiveView-owned async handling (start_async/3 plus handle_async/3, or an equivalent pattern that sends results back to the LiveView process). Do not use bare Task.start/1, because it cannot update the socket, reinsert the stream item, or show an error toast.

Expected behavior:

1. Fetch the item needed for rollback.
2. Immediately call stream_delete/3 on the relevant stream.
3. Start an async delete operation owned by the LiveView.
4. On success, leave the stream as-is and optionally broadcast/refresh any cross-view state the existing design requires.
5. On failure or async exit, reinsert the original item into the stream and show a user-facing error toast.

For error text, use the project's existing user-facing error conventions (ErrorMessages.friendly_message/1 where an error reason is shown). For {:exit, reason}, use a generic gettext string — the exit reason may contain stacktraces.

handle_async signatures

Per project conventions, handle_async always handles three cases:

• {:ok, {:ok, result}} — delete succeeded, stream already clean, no-op
• {:ok, {:error, reason}} — delete returned error, reinsert + toast with ErrorMessages.friendly_message/1
• {:exit, reason} — task crashed, reinsert + generic error toast (do NOT leak reason to user)

Acceptance Criteria

• #1 Index/list delete operations remove the item from the relevant stream before waiting for the database delete
• #2 Database delete still executes through LiveView-owned async handling, not bare Task.start/1
• #3 Async delete failure or exit reinserts the original item into the stream and shows an error toast
• #4 Existing delete confirmation prompts still work correctly
• #5 Show-page delete handlers are not changed as part of this task
• #6 Collection and wishlist index delete behavior remains shared through IndexActions where practical
• #7 Existing data-confirm browser prompts are not bypassed or altered
• #8 handle_async {:exit, reason} uses a generic gettext string, never leaks stacktrace reason to user
• #9 All three LiveViews have delete tests covering success, error, and exit paths

Implementation Plan

Implementation Plan

1. IndexActions.handle_delete/2 — shared path (collection + wishlist)

Replace the synchronous delete with optimistic stream_delete + start_async:

def handle_delete(socket, id) do
  record = Records.get_record!(id)
  socket = stream_delete(socket, :records, record)
  {:noreply, start_async(socket, {:delete_record, id}, fn ->
    Records.delete_record(record)
  end)}
end

Key: {:delete_record, id} as async key ensures uniqueness. The async closure captures record — the LiveView task supervisor holds the reference. Do NOT call load_and_assign_records — stream_delete already removes the item from the stream; a full reload would negate the performance benefit.

2. CollectionLive.Index — handle_async callbacks

Add three handle_async clauses after the existing handle_async(:collection_summary, ...) clauses:

@impl true
def handle_async({:delete_record, id}, {:ok, {:ok, _record}}, socket) do
  {:noreply, socket}
end

def handle_async({:delete_record, id}, {:ok, {:error, reason}}, socket) do
  record = Records.get_record!(id)
  {:noreply,
   socket
   |> stream_insert(:records, record)
   |> put_toast(:error, gettext("Failed to delete: ") <> ErrorMessages.friendly_message(reason))}
end

def handle_async({:delete_record, id}, {:exit, _reason}, socket) do
  record = Records.get_record!(id)
  {:noreply,
   socket
   |> stream_insert(:records, record)
   |> put_toast(:error, gettext("Delete failed, please try again"))}
end

Note: Both error cases re-fetch via Records.get_record!(id). For {:error, reason}, the record was NOT deleted so re-fetch succeeds. For {:exit, _reason}, the task crashed — if the record was deleted before the crash, get_record! raises Ecto.NoResultsError which crashes the LiveView (acceptable: better than silently showing wrong state).

3. WishlistLive.Index — identical handle_async callbacks

Duplicate the same three handle_async clauses from CollectionLive.Index. Per project conventions, extraction at 2 callers is premature; extract to IndexActions only when a third index page needs it.

4. ScrobbledTracksLive.Index — separate async path

Replace the synchronous delete handler:

def handle_event("delete", %{"scrobbled-at-uts" => scrobbled_at_uts}, socket) do
  track = ListeningStats.get_track!(scrobbled_at_uts)
  stream_element = %{track: track}
  socket = stream_delete(socket, :tracks, stream_element)

  {:noreply, start_async(socket, {:delete_track, scrobbled_at_uts}, fn ->
    {ListeningStats.delete_track(track), stream_element}
  end)}
end

Add three handle_async clauses:

@impl true
def handle_async({:delete_track, _uts}, {:ok, {{:ok, _track}, _element}}, socket) do
  {:noreply, socket}
end

def handle_async({:delete_track, _uts}, {:ok, {{:error, reason}, element}}, socket) do
  {:noreply,
   socket
   |> stream_insert(:tracks, element)
   |> put_toast(:error, gettext("Failed to delete: ") <> ErrorMessages.friendly_message(reason))}
end

def handle_async({:delete_track, _uts}, {:exit, _reason}, socket) do
  {:noreply, put_toast(socket, :error, gettext("Delete failed, please try again"))}
end

Key differences from records path:

• Passes stream_element through async tuple so rollback uses the exact same map shape without re-running correlated subqueries (audit Finding 7)
• {:exit, _reason} cannot recover the stream element — accept this limitation and show error toast only. A full load_and_assign_tracks reload would trigger the expensive correlated subqueries.

5. Tests

Currently zero delete test coverage across all three index test files. Add:

#	Test	File
1	Delete record from collection grid (success)	collection_live/index_test.exs
2	Delete record from wishlist (success)	wishlist_live/index_test.exs
3	Delete scrobbled track (success)	scrobbled_tracks_live/index_test.exs
4	handle_async {:ok, {:ok, ...}} is no-op	collection_live/index_test.exs (unit test)
5	handle_async {:ok, {:error, ...}} reinserts + toasts	collection_live/index_test.exs (unit test)
6	handle_async {:exit, ...} reinserts + toasts	collection_live/index_test.exs (unit test)
7	handle_async exit uses generic message (not reason)	collection_live/index_test.exs (unit test)
8	data-confirm attribute present on delete buttons	collection_live/index_test.exs (AC #4 regression)
9	Show page delete unchanged (still synchronous)	collection_live/show_test.exs (AC #5 regression)

Integration tests (tests 1-3): click delete button inside dropdown, verify refute_has for the record/track ID, verify record is actually deleted from DB via context module.

Unit tests (tests 4-7): import the LiveView module, call handle_async/3 directly with simulated results, assert socket state (stream contents, toast message). This is the preferred approach for testing failure paths since mocking Repo.delete is invasive.

Test 7 specifically asserts that {:exit, :killed} produces a generic gettext string, not the raw exit reason — this prevents accidental stacktrace leakage to users.