Simplify preloading in BoardListType.title

https://gitlab.com/gitlab-org/gitlab/-/issues/342889

app/graphql/types/board_list_type.rb

@@ -10,8 +10,10 @@ module Types
 
     alias_method :list, :object
 
-    field :id, GraphQL::Types::ID, null: false,
+    field :id, GraphQL::Types::ID,
+          null: false,
           description: 'ID (global ID) of the list.'
+
     field :title, GraphQL::Types::String, null: false,
           description: 'Title of the list.'
     field :list_type, GraphQL::Types::String, null: false,
@@ -50,14 +52,11 @@ module Types
 
     # board lists have a data dependency on label - so we batch load them here
     def title
-      if object.association(:label).loaded? && object.label_id.present?
-        object.title
-      else
-        loader = Gitlab::Graphql::Loaders::BatchModelLoader.new(Label, object.label_id)
-        Gitlab::Graphql::Lazy.with_value(loader.find) do |label|
-          object.label = label
-          object.title
-        end
+      BatchLoader::GraphQL.for(object).batch do |lists, callback|
+        ActiveRecord::Associations::Preloader.new.preload(lists, :label) # rubocop: disable CodeReuse/ActiveRecord
+
+        # all list titles are preloaded at this point
+        lists.each { |list| callback.call(list, list.title) }
       end
     end
   end

spec/graphql/types/board_list_type_spec.rb

@@ -3,10 +3,13 @@
 require 'spec_helper'
 
 RSpec.describe GitlabSchema.types['BoardList'] do
+  include GraphqlHelpers
+  include Gitlab::Graphql::Laziness
+
   specify { expect(described_class.graphql_name).to eq('BoardList') }
 
   it 'has specific fields' do
-    expected_fields = %w[id list_type position label issues_count issues]
+    expected_fields = %w[id title list_type position label issues_count issues]
 
     expect(described_class).to include_graphql_fields(*expected_fields)
   end
@@ -18,4 +21,18 @@ RSpec.describe GitlabSchema.types['BoardList'] do
       is_expected.to have_graphql_extension(Gitlab::Graphql::Board::IssuesConnectionExtension)
     end
   end
+
+  describe 'title' do
+    subject(:field) { described_class.fields['title'] }
+
+    it 'preloads the label association' do
+      a, b, c = create_list(:list, 3).map { _1.class.find(_1.id) }
+
+      baseline = ActiveRecord::QueryRecorder.new { force(resolve_field(field, a)) }
+
+      expect do
+        [resolve_field(field, b), resolve_field(field, c)].each { force _1 }
+      end.not_to exceed_query_limit(baseline)
+    end
+  end
 end