GraphQL - Preload epics group using Lookahead

ee/app/graphql/ee/types/group_type.rb

@@ -18,6 +18,7 @@ module EE
 
         field :epics, ::Types::EpicType.connection_type, null: true,
               description: 'Find epics',
+              extras: [:lookahead],
               max_page_size: 2000,
               resolver: ::Resolvers::EpicsResolver
 

ee/app/graphql/resolvers/epics_resolver.rb

@@ -3,6 +3,7 @@
 module Resolvers
   class EpicsResolver < BaseResolver
     include TimeFrameArguments
+    include LooksAhead
 
     argument :iid, GraphQL::ID_TYPE,
              required: false,
@@ -49,7 +50,7 @@ module Resolvers
       super(args)
     end
 
-    def resolve(**args)
+    def resolve_with_lookahead(**args)
       @resolver_object = object.respond_to?(:sync) ? object.sync : object
 
       return [] unless resolver_object.present?
@@ -62,8 +63,12 @@ module Resolvers
 
     attr_reader :resolver_object
 
+    def unconditional_includes
+      [:group]
+    end
+
     def find_epics(args)
-      EpicsFinder.new(context[:current_user], args).execute
+      apply_lookahead(EpicsFinder.new(context[:current_user], args).execute)
     end
 
     def epic_feature_enabled?
@@ -72,7 +77,7 @@ module Resolvers
 
     def transform_args(args)
       transformed               = args.dup
-      transformed[:group_id]    = group.id
+      transformed[:group_id]    = group
       transformed[:parent_id]   = parent.id if parent
       transformed[:iids]      ||= [args[:iid]].compact
 

ee/app/graphql/types/epic_type.rb

@@ -30,8 +30,7 @@ module Types
           description: 'Indicates if the epic is confidential'
 
     field :group, GroupType, null: false,
-          description: 'Group to which the epic belongs',
-          resolve: -> (obj, _args, _ctx) { Gitlab::Graphql::Loaders::BatchModelLoader.new(Group, obj.group_id).find }
+          description: 'Group to which the epic belongs'
     field :parent, EpicType, null: true,
           description: 'Parent epic of the epic',
           resolve: -> (obj, _args, _ctx) { Gitlab::Graphql::Loaders::BatchModelLoader.new(Epic, obj.parent_id).find }

ee/changelogs/unreleased/210553-improve-performance-of-graphql-groupepics-query.yml

+---
+title: Preload epics in GraphQL group queries using Lookahead
+merge_request: 42874
+author:
+type: performance

ee/spec/requests/api/graphql/group_query_spec.rb

@@ -196,5 +196,42 @@ RSpec.describe 'getting group information' do
         expect(graphql_errors).to eq([nil, nil])
       end
     end
+
+    context 'when loading multiple epics' do
+      let_it_be(:group) { create(:group) }
+
+      before do
+        stub_licensed_features(epics: true)
+        query_epics(1)
+      end
+
+      it 'can lookahead to eliminate N+1 queries', :use_clean_rails_memory_store_caching do
+        create_list(:epic, 10, group: group)
+        group.reload
+
+        control_count = ActiveRecord::QueryRecorder.new(skip_cached: false) do
+          query_epics(1)
+        end.count
+
+        expect { query_epics(10) }.not_to exceed_all_query_limit(control_count)
+      end
+    end
+
+    def query_epics(number)
+      epics_field = <<~NODE
+        epics(first: #{number}) {
+          edges {
+            node {
+              title
+            }
+          }
+        }
+      NODE
+
+      post_graphql(
+        graphql_query_for('group', { 'fullPath' => group.full_path }, epics_field),
+        current_user: user
+      )
+    end
   end
 end