ML-190: fix redundant unsubscribe (plan)

backlog/tasks/ml-190 - Skip-redundant-unsubscriberesubscribe-on-same-record-reconnect.md

@@ -4,6 +4,7 @@ title: Skip redundant unsubscribe+resubscribe on same-record reconnect
 status: To Do
 assignee: []
 created_date: "2026-05-19 08:42"
+updated_date: "2026-05-19 09:26"
 labels:
   - audit
   - pubsub
@@ -39,3 +40,72 @@ ordinal: 25000
 - [ ] #2 Same-record reconnect skips unsubscribe+resubscribe entirely
 - [ ] #3 All existing tests pass
 <!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+
+## Overview
+
+Micro-optimization to skip the redundant `Records.unsubscribe/1` call when a LiveView reconnects to the same record. The change is a single guard addition in `RecordActions.manage_subscription/2`. No other modules are affected.
+
+**Nature:** Code hygiene, not a measurable performance win. The skip eliminates at most two ETS operations on reconnect, which Phoenix.PubSub already deduplicates by PID internally.
+
+## Callers (no changes needed)
+
+| LiveView            | File                                                 | Line | Hook              |
+| ------------------- | ---------------------------------------------------- | ---- | ----------------- |
+| CollectionLive.Show | `lib/music_library_web/live/collection_live/show.ex` | 361  | `handle_params/3` |
+| WishlistLive.Show   | `lib/music_library_web/live/wishlist_live/show.ex`   | 298  | `handle_params/3` |
+
+Both call `manage_subscription(socket, id)` at the top of `handle_params/3` before loading the record. No caller changes are needed because the function signature and return value (`:ok`) remain unchanged.
+
+## Code change
+
+**File:** `lib/music_library_web/live_helpers/record_actions.ex`
+
+Change the unsubscribe guard from:
+
+```elixir
+if socket.assigns[:record], do: Records.unsubscribe(socket.assigns.record.id)
+```
+
+to:
+
+```elixir
+if socket.assigns[:record] && socket.assigns.record.id != new_id,
+  do: Records.unsubscribe(socket.assigns.record.id)
+```
+
+The `Records.subscribe(new_id)` call is intentionally left unconditional: on same-record reconnect it's a no-op (PubSub deduplicates by PID), and on first mount it's required.
+
+## Edge-case coverage
+
+| Scenario                     | `socket.assigns[:record]` | old vs new     | Unsub? | Sub?        | Correct? |
+| ---------------------------- | ------------------------- | -------------- | ------ | ----------- | -------- |
+| First mount                  | `nil`                     | short-circuits | No     | Yes         | ✅       |
+| Navigate to different record | record A                  | A ≠ B          | Yes    | Yes         | ✅       |
+| Reconnect same record        | record X                  | X == X         | No     | Yes (no-op) | ✅       |
+
+## Verification steps
+
+1. **Run existing tests:**
+
+   ```bash
+   mix test test/music_library_web/live_helpers/record_actions_test.exs
+   mix test test/music_library_web/live/collection_live/show_test.exs
+   mix test test/music_library_web/live/wishlist_live/show_test.exs
+   ```
+
+   All should pass. These cover `manage_subscription/2` indirectly through PubSub broadcast handling and full page navigation tests.
+
+2. **Manual reconnect smoke test (optional):** Navigate to a record show page, kill the server, restart it, and confirm the page reconnects and continues to receive PubSub updates (e.g., trigger a background cover refresh and verify the toast appears).
+
+3. **No new test is strictly required.** The change is a pure optimization — it removes a redundant call that was already harmless. Existing PubSub broadcast tests in `record_actions_test.exs` (the `handle_record_updated` describe block) confirm the subscription lifecycle works end-to-end.
+
+## Documentation
+
+- No user-facing docs need updating.
+- The `@doc` for `manage_subscription/2` is already accurate and does not need amendment.
+- No production infrastructure changes, environment variables, or migrations are required.
+<!-- SECTION:PLAN:END -->