ML-190: fix record subscription lifecycle
This commit is contained in:
Claudio Ortolina
+51
-65
@@ -1,10 +1,11 @@
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---
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id: ML-190
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title: Skip redundant unsubscribe+resubscribe on same-record reconnect
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status: To Do
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assignee: []
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status: Done
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assignee:
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- Codex
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created_date: "2026-05-19 08:42"
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updated_date: "2026-05-19 09:26"
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updated_date: "2026-05-19 11:06"
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labels:
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- audit
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- pubsub
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@@ -16,6 +17,7 @@ documentation:
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Audit-Report-PubSub-Subscription-Lifecycle-Phase-2.md
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modified_files:
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- lib/music_library_web/live_helpers/record_actions.ex
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- test/music_library_web/live_helpers/record_actions_test.exs
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priority: low
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ordinal: 25000
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---
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@@ -24,11 +26,11 @@ ordinal: 25000
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<!-- SECTION:DESCRIPTION:BEGIN -->
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`RecordActions.manage_subscription/2` unsubscribes the old record and subscribes the new record on every `handle_params` call. When LiveView reconnects to the same record (e.g., after a WebSocket drop within the reconnect grace period), it unsubscribes and resubscribes to the same topic. While harmless (Phoenix.PubSub deduplicates by PID), a same-record check would be a micro-optimization.
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`RecordActions.manage_subscription/2` manages record show PubSub topics from `handle_params/3`. Phoenix.PubSub allows duplicate subscriptions for the same PID/topic and delivers duplicate events, so same-record parameter handling must leave the existing subscription untouched instead of subscribing again.
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**Fix:** Add a guard: `if socket.assigns[:record] && socket.assigns.record.id != new_id, do: Records.unsubscribe(...)` — only unsubscribe when the record actually changes.
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**Fix:** Make `manage_subscription/2` distinguish first subscription, different-record navigation, and same-record no-op. First mount subscribes to the record topic; navigating to a different record unsubscribes the old topic and subscribes the new topic; same-record handling does nothing.
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**Source:** Audit doc-25 (Phase 2), Recommendation #2.
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**Source:** Audit doc-25 (Phase 2), Recommendation #2, corrected after verifying Phoenix.PubSub duplicate-subscription behavior.
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<!-- SECTION:DESCRIPTION:END -->
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@@ -36,9 +38,10 @@ ordinal: 25000
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<!-- AC:BEGIN -->
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- [ ] #1 manage_subscription/2 only calls unsubscribe when old record ID differs from new ID
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- [ ] #2 Same-record reconnect skips unsubscribe+resubscribe entirely
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- [ ] #3 All existing tests pass
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- [x] #1 manage_subscription/2 leaves the existing PubSub subscription untouched when the assigned record ID already matches the new ID
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- [x] #2 manage_subscription/2 still unsubscribes from the old record and subscribes to the new record when navigating between different records
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- [x] #3 A regression test verifies same-record parameter handling does not create duplicate PubSub deliveries
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- [x] #4 Relevant tests pass
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<!-- AC:END -->
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## Implementation Plan
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@@ -47,65 +50,48 @@ ordinal: 25000
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## Overview
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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.
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Correct the same-record subscription optimization so it avoids both sides of the redundant PubSub lifecycle. Phoenix.PubSub allows duplicate subscriptions for the same PID/topic and will deliver duplicate events, so same-record handling must not call `Records.subscribe/1` again.
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**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.
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## Implementation
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## Callers (no changes needed)
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| LiveView | File | Line | Hook |
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| ------------------- | ---------------------------------------------------- | ---- | ----------------- |
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| CollectionLive.Show | `lib/music_library_web/live/collection_live/show.ex` | 361 | `handle_params/3` |
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| WishlistLive.Show | `lib/music_library_web/live/wishlist_live/show.ex` | 298 | `handle_params/3` |
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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.
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## Code change
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**File:** `lib/music_library_web/live_helpers/record_actions.ex`
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Change the unsubscribe guard from:
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```elixir
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if socket.assigns[:record], do: Records.unsubscribe(socket.assigns.record.id)
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```
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to:
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```elixir
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if socket.assigns[:record] && socket.assigns.record.id != new_id,
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do: Records.unsubscribe(socket.assigns.record.id)
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```
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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.
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## Edge-case coverage
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| Scenario | `socket.assigns[:record]` | old vs new | Unsub? | Sub? | Correct? |
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| ---------------------------- | ------------------------- | -------------- | ------ | ----------- | -------- |
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| First mount | `nil` | short-circuits | No | Yes | ✅ |
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| Navigate to different record | record A | A ≠ B | Yes | Yes | ✅ |
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| Reconnect same record | record X | X == X | No | Yes (no-op) | ✅ |
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## Verification steps
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1. **Run existing tests:**
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```bash
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mix test test/music_library_web/live_helpers/record_actions_test.exs
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mix test test/music_library_web/live/collection_live/show_test.exs
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mix test test/music_library_web/live/wishlist_live/show_test.exs
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```
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All should pass. These cover `manage_subscription/2` indirectly through PubSub broadcast handling and full page navigation tests.
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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).
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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.
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1. Update `lib/music_library_web/live_helpers/record_actions.ex` so `manage_subscription/2` has explicit branches for first subscription, different-record navigation, and same-record no-op.
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2. Add focused regression tests in `test/music_library_web/live_helpers/record_actions_test.exs` using a connected `Phoenix.LiveView.Socket` and the real `Records.subscribe/1` / `Records.notify_update/1` PubSub path.
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3. Verify the same-record case delivers one broadcast once, and the different-record case stops receiving the old topic while receiving the new topic.
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4. Run the focused helper/show tests and format the changed files.
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5. Update this Backlog task to replace the previous incorrect PubSub deduplication note with the verified behavior.
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## Documentation
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- No user-facing docs need updating.
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- The `@doc` for `manage_subscription/2` is already accurate and does not need amendment.
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- No production infrastructure changes, environment variables, or migrations are required.
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No project architecture, production infrastructure, or convention docs need codebase-level changes because this is a bug fix within the existing `LiveHelpers.RecordActions.manage_subscription/2` pattern. The necessary documentation updates are the local `@doc` for `manage_subscription/2` and this task record, since it previously claimed Phoenix.PubSub deduplicates duplicate subscriptions.
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<!-- SECTION:PLAN:END -->
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## Implementation Notes
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<!-- SECTION:NOTES:BEGIN -->
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Found that Phoenix.PubSub duplicate subscriptions are allowed and produce duplicate deliveries; the previous task notes/final summary claiming PID/topic deduplication were incorrect.
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Updated `manage_subscription/2` to treat same-record handling as a no-op, preserving first-subscribe and different-record switch behavior. Added regression tests that prove one broadcast is delivered once after same-record handling and that different-record navigation drops the old topic.
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<!-- SECTION:NOTES:END -->
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## Final Summary
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<!-- SECTION:FINAL_SUMMARY:BEGIN -->
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Updated `RecordActions.manage_subscription/2` so it no-ops when the assigned record ID already matches the requested record ID. This prevents duplicate Phoenix.PubSub subscriptions, which would otherwise deliver duplicate `{:update, record}` messages for one broadcast.
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The helper now has explicit branches for first subscription, different-record navigation, and same-record no-op. Its `@doc` was corrected to describe the actual behavior.
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Added focused regression coverage in `test/music_library_web/live_helpers/record_actions_test.exs`:
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- same-record subscription handling does not duplicate PubSub deliveries
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- navigating between records unsubscribes the old topic and subscribes the new topic
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Documentation update: corrected this Backlog task's description, plan, notes, and final summary. No project architecture, production infrastructure, or convention docs required changes because the behavior remains within the existing LiveView subscription pattern.
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Tests run:
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`mix test test/music_library_web/live_helpers/record_actions_test.exs test/music_library_web/live/collection_live/show_test.exs test/music_library_web/live/wishlist_live/show_test.exs` -> 23 passed
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<!-- SECTION:FINAL_SUMMARY:END -->
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@@ -99,13 +99,22 @@ defmodule MusicLibraryWeb.LiveHelpers.RecordActions do
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@doc """
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Manages PubSub subscriptions when navigating between records.
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Unsubscribes from the previous record (if any) and subscribes to the new one,
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but only when the socket is connected.
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Subscribes on first connect, switches topics when the record changes, and
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leaves same-record subscriptions untouched to avoid duplicate PubSub events.
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"""
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def manage_subscription(socket, new_id) do
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if Phoenix.LiveView.connected?(socket) do
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if socket.assigns[:record], do: Records.unsubscribe(socket.assigns.record.id)
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Records.subscribe(new_id)
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case socket.assigns[:record] do
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%{id: ^new_id} ->
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:ok
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%{id: old_id} ->
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Records.unsubscribe(old_id)
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Records.subscribe(new_id)
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_missing_record ->
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Records.subscribe(new_id)
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end
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end
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:ok
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@@ -9,6 +9,7 @@ defmodule MusicLibraryWeb.LiveHelpers.RecordActionsTest do
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alias MusicLibrary.Chats
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alias MusicLibrary.Records
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alias MusicLibrary.Records.Similarity
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alias MusicLibraryWeb.LiveHelpers.RecordActions
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alias Req.Test
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# A MusicBrainz stub that returns valid fixture responses for every route
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@@ -71,6 +72,49 @@ defmodule MusicLibraryWeb.LiveHelpers.RecordActionsTest do
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end)
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end
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defp connected_socket(assigns \\ %{}) do
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%Phoenix.LiveView.Socket{transport_pid: self(), assigns: assigns}
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end
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describe "manage_subscription" do
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test "does not duplicate a same-record subscription" do
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record = record()
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assert :ok = RecordActions.manage_subscription(connected_socket(), record.id)
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assert :ok =
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RecordActions.manage_subscription(connected_socket(%{record: record}), record.id)
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updated_record = %{record | title: "Single PubSub Delivery Title"}
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assert :ok = Records.notify_update(updated_record)
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assert_receive {:update, ^updated_record}
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refute_receive {:update, ^updated_record}, 100
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end
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test "switches subscriptions when navigating between records" do
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old_record = record()
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new_record = record()
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assert :ok = RecordActions.manage_subscription(connected_socket(), old_record.id)
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assert :ok =
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RecordActions.manage_subscription(
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connected_socket(%{record: old_record}),
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new_record.id
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)
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updated_old_record = %{old_record | title: "Old Record Update"}
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updated_new_record = %{new_record | title: "New Record Update"}
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assert :ok = Records.notify_update(updated_old_record)
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refute_receive {:update, ^updated_old_record}, 100
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assert :ok = Records.notify_update(updated_new_record)
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assert_receive {:update, ^updated_new_record}
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end
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end
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describe "refresh_musicbrainz_data event" do
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test "success path shows confirmation toast", %{conn: conn} do
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record = record()
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