Merge pull request #312: Find the messages a refresh replaces through an index

Reviewed and merged by GPT on behalf of DHH.

app/models/message/pagination.rb

@@ -36,7 +36,10 @@ module Message::Pagination
     scope :page_after, ->(message) { after(message).first_page }
 
     scope :page_created_since, ->(time) { where("created_at > ?", time).first_page }
-    scope :page_updated_since, ->(time) { where("updated_at > ?", time).last_page }
+    # Sorting on +created_at, which no index covers, has SQLite find the few messages updated since
+    # through (room_id, updated_at) and sort just those, instead of walking the whole room by
+    # (room_id, created_at) looking for them.
+    scope :page_updated_since, ->(time) { Page.load(where("updated_at > ?", time).reorder(Arel.sql("+messages.created_at")), :last, PAGE_SIZE) }
   end
 
   class_methods do

db/migrate/20261005022000_add_room_id_and_updated_at_index_to_messages.rb

@@ -0,0 +1,5 @@
+class AddRoomIdAndUpdatedAtIndexToMessages < ActiveRecord::Migration[8.2]
+  def change
+    add_index :messages, %i[ room_id updated_at ], if_not_exists: true
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.2].define(version: 2026_10_04_225000) do
+ActiveRecord::Schema[8.2].define(version: 2026_10_05_022000) do
   create_table "accounts", force: :cascade do |t|
     t.datetime "created_at", null: false
     t.text "custom_styles"
@@ -102,6 +102,7 @@ ActiveRecord::Schema[8.2].define(version: 2026_10_04_225000) do
     t.datetime "updated_at", null: false
     t.index ["creator_id"], name: "index_messages_on_creator_id"
     t.index ["room_id", "created_at"], name: "index_messages_on_room_id_and_created_at"
+    t.index ["room_id", "updated_at"], name: "index_messages_on_room_id_and_updated_at"
     t.index ["room_id"], name: "index_messages_on_room_id"
   end
 

test/models/message/pagination_test.rb

@@ -0,0 +1,16 @@
+require "test_helper"
+
+class Message::PaginationTest < ActiveSupport::TestCase
+  test "messages updated since a time are the newest page of them, in creation order" do
+    room = rooms(:watercooler)
+    first, second, third = room.messages.ordered.first(3)
+
+    travel 1.minute do
+      [ third, first, second ].each(&:touch)
+
+      stub_const(Message::Pagination, :PAGE_SIZE, 2) do
+        assert_equal [ second, third ], room.messages.page_updated_since(30.seconds.ago).to_a
+      end
+    end
+  end
+end