fix: preserve pair chat when helpers rejoin

lib/who_need_help/messaging.ex

@@ -28,10 +28,11 @@ defmodule WhoNeedHelp.Messaging do
       limit = Pagination.limit(options, 50)
       cursor = Pagination.cursor(options)
       public_user = Accounts.public_user_query()
+      assignment_ids = conversation_assignment_ids(assignment)
 
       page =
         Message
-        |> where([message], message.assignment_id == ^assignment.id)
+        |> where([message], message.assignment_id in subquery(assignment_ids))
         |> before_message(cursor)
         |> order_by([message], desc: message.inserted_at, desc: message.id)
         |> limit(^(limit + 1))
@@ -112,10 +113,11 @@ defmodule WhoNeedHelp.Messaging do
     if Trust.eligible?(scope) and Help.participant?(scope, assignment) and
          not blocked_assignment?(scope, assignment) do
       now = DateTime.utc_now(:second)
+      assignment_ids = conversation_assignment_ids(assignment)
 
       {count, _messages} =
         Message
-        |> where([message], message.assignment_id == ^assignment.id)
+        |> where([message], message.assignment_id in subquery(assignment_ids))
         |> where([message], message.sender_id != ^user.id)
         |> where([message], is_nil(message.read_at))
         |> Repo.update_all(set: [read_at: now, updated_at: now])
@@ -139,6 +141,16 @@ defmodule WhoNeedHelp.Messaging do
     Trust.blocked_between?(user.id, counterpart_id(user.id, assignment, request))
   end
 
+  defp conversation_assignment_ids(%Assignment{
+         request_id: request_id,
+         helper_id: helper_id
+       }) do
+    from(current in Assignment,
+      where: current.request_id == ^request_id and current.helper_id == ^helper_id,
+      select: current.id
+    )
+  end
+
   defp counterpart_id(user_id, assignment, request) do
     if user_id == assignment.helper_id, do: request.requester_id, else: assignment.helper_id
   end

priv/repo/migrations/20260723015032_add_help_assignment_request_helper_index.exs

@@ -0,0 +1,7 @@
+defmodule WhoNeedHelp.Repo.Migrations.AddHelpAssignmentRequestHelperIndex do
+  use Ecto.Migration
+
+  def change do
+    create index(:help_assignments, [:request_id, :helper_id])
+  end
+end

test/who_need_help/mutual_aid_flow_test.exs

@@ -269,6 +269,43 @@ defmodule WhoNeedHelp.MutualAidFlowTest do
              Messaging.send_message(outsider_scope, assignment, %{"body" => "not allowed"})
   end
 
+  test "the same helper keeps pair chat after rejoining without exposing it to a replacement",
+       context do
+    replacement = user_fixture(display_name: "Replacement helper")
+    replacement_scope = user_scope_fixture(replacement)
+
+    {:ok, request} = Help.create_request(context.requester_scope, context.request_attrs)
+    {:ok, first_assignment} = Help.accept_request(context.helper_scope, request.id)
+
+    {:ok, first_message} =
+      Messaging.send_message(context.helper_scope, first_assignment, %{
+        "body" => "I can collect this order."
+      })
+
+    assert {:ok, _withdrawn} =
+             Help.withdraw_assignment(context.helper_scope, first_assignment.id)
+
+    assert {:ok, rejoined_assignment} = Help.accept_request(context.helper_scope, request.id)
+    assert rejoined_assignment.id != first_assignment.id
+
+    assert [%{id: message_id, body: "I can collect this order."}] =
+             Messaging.list_messages(context.requester_scope, rejoined_assignment)
+
+    assert message_id == first_message.id
+
+    assert [%{id: ^message_id}] =
+             Messaging.list_messages(context.helper_scope, rejoined_assignment)
+
+    assert {:ok, _withdrawn_again} =
+             Help.withdraw_assignment(context.helper_scope, rejoined_assignment.id)
+
+    assert {:ok, replacement_assignment} =
+             Help.accept_request(replacement_scope, request.id)
+
+    assert [] == Messaging.list_messages(context.requester_scope, replacement_assignment)
+    assert [] == Messaging.list_messages(replacement_scope, replacement_assignment)
+  end
+
   test "cancelling in-progress help closes interaction and deletes live positions", context do
     {:ok, request} = Help.create_request(context.requester_scope, context.request_attrs)
     {:ok, assignment} = Help.accept_request(context.helper_scope, request.id)