Speed up the card list for large accounts

app/models/filter.rb

@@ -27,7 +27,7 @@ def cards
       result = result.unassigned if assignment_status.unassigned?
       result = result.assigned_to(assignees.ids) if assignees.present?
       result = result.where(creator_id: creators.ids) if creators.present?
-      result = result.where(board: boards.ids) if boards.present?
+      result = filter_boards(result) if boards.present?
       result = result.tagged_with(tags.ids) if tags.present?
       result = result.where(cards: { created_at: creation_window }) if creation_window
       result = result.closed_at_window(closure_window) if closure_window
@@ -66,6 +66,23 @@ def only_closed?
   end
 
   private
+    def filter_boards(relation)
+      relation = relation.where(cards: { account_id: creator.account_id }).where(board: boards.ids)
+      if joins_has_many?
+        relation
+      else
+        # Pin the (account_id, last_active_at, status) index so the ordered page is served by a reverse scan, not a filesort.
+        relation.use_index(:index_cards_on_account_id_and_last_active_at_and_status)
+      end
+    end
+
+    # Assignee, tag, and term filters add a has-many join that fans a card into
+    # several rows; the ordered reverse scan loses to a different plan then, so
+    # the pin only applies without them.
+    def joins_has_many?
+      assignees.present? || tags.present? || terms.present?
+    end
+
     def include_closed_cards?
       only_closed? || card_ids.present?
     end

test/models/filter_test.rb

@@ -48,6 +48,15 @@ class FilterTest < ActiveSupport::TestCase
     assert_empty users(:david).filters.new(board_ids: [ boards(:writebook).id ]).boards
   end
 
+  test "board-scoped cards never leak cards from an inaccessible board in the same account" do
+    inaccessible_card = boards(:private).cards.create!(status: "published", creator: users(:kevin))
+
+    filter = users(:david).filters.new(board_ids: [ boards(:private).id, boards(:writebook).id ])
+
+    assert_not_includes filter.cards, inaccessible_card
+    filter.cards.each { |card| assert_includes users(:david).boards, card.board }
+  end
+
   test "remembering equivalent filters" do
     assert_difference "Filter.count", +1 do
       filter = users(:david).filters.remember(sorted_by: "latest", assignment_status: "unassigned", tag_ids: [ tags(:mobile).id ])