Add option to query a single board list on GraphQL

app/graphql/resolvers/board_lists_resolver.rb

@@ -6,12 +6,16 @@ module Resolvers
 
     type Types::BoardListType, null: true
 
+    argument :id, GraphQL::ID_TYPE,
+             required: false,
+             description: 'Find a list by its ID'
+
     alias_method :board, :object
 
-    def resolve(lookahead: nil)
+    def resolve(lookahead: nil, id: nil)
       authorize!(board)
 
-      lists = board_lists
+      lists = board_lists(id)
 
       if load_preferences?(lookahead)
         List.preload_preferences_for_user(lists, context[:current_user])
@@ -22,8 +26,8 @@ module Resolvers
 
     private
 
-    def board_lists
-      service = Boards::Lists::ListService.new(board.resource_parent, context[:current_user])
+    def board_lists(id)
+      service = Boards::Lists::ListService.new(board.resource_parent, context[:current_user], list_id: extract_list_id(id))
       service.execute(board, create_default_lists: false)
     end
 
@@ -34,5 +38,11 @@ module Resolvers
     def load_preferences?(lookahead)
       lookahead&.selection(:edges)&.selection(:node)&.selects?(:collapsed)
     end
+
+    def extract_list_id(gid)
+      return unless gid.present?
+
+      GitlabSchema.parse_gid(gid, expected_type: ::List).model_id
+    end
   end
 end

app/services/boards/lists/list_service.rb

@@ -8,7 +8,8 @@ module Boards
           board.lists.create(list_type: :backlog)
         end
 
-        board.lists.preload_associated_models
+        lists = board.lists.preload_associated_models
+        params[:list_id].present? ? lists.where(id: params[:list_id]) : lists # rubocop: disable CodeReuse/ActiveRecord
       end
     end
   end

spec/graphql/resolvers/board_lists_resolver_spec.rb

@@ -57,6 +57,25 @@ RSpec.describe Resolvers::BoardListsResolver do
           expect(lists.count).to eq 3
         end
       end
+
+      context 'when querying for a single list' do
+        it 'returns specified list' do
+          list = resolve_board_lists(args: { id: global_id_of(label_list) }).items
+
+          expect(list).to eq [label_list]
+        end
+
+        it 'returns empty result if list is not accessible found' do
+          external_group = create(:group, :private)
+          external_board = create(:board, resource_parent: external_group )
+          external_label = create(:group_label, group: group)
+          external_list = create(:list, board: external_board, label: external_label)
+
+          list = resolve_board_lists(args: { id: global_id_of(external_list) }).items
+
+          expect(list).to eq List.none
+        end
+      end
     end
   end