Strongly memoize list

- Add N+1 test

app/services/boards/issues/list_service.rb

@@ -55,9 +55,17 @@ module Boards
       end
 
       def list
-        return @list if defined?(@list)
+        return unless params.key?(:id)
 
-        @list = board.lists.find(params[:id]) if params.key?(:id)
+        strong_memoize(:list) do
+          id = params[:id]
+
+          if board.lists.loaded?
+            board.lists.find { |l| l.id == id }
+          else
+            board.lists.find(id)
+          end
+        end
       end
 
       def filter_params

spec/support/shared_examples/services/boards/issues_list_service_shared_examples.rb

@@ -19,6 +19,15 @@ RSpec.shared_examples 'issues list service' do
     end
   end
 
+  it 'avoids N+1' do
+    params = { board_id: board.id }
+    control = ActiveRecord::QueryRecorder.new { described_class.new(parent, user, params).execute }
+
+    create(:list, board: board)
+
+    expect { described_class.new(parent, user, params).execute }.not_to exceed_query_limit(control)
+  end
+
   context 'issues are ordered by priority' do
     it 'returns opened issues when list_id is missing' do
       params = { board_id: board.id }