Preload parent in group epics query

- Use Lookahead to preload the field parent
when querying group epics using GraphQL

ee/app/graphql/resolvers/epics_resolver.rb

@@ -67,6 +67,12 @@ module Resolvers
       [:group]
     end
 
+    def preloads
+      {
+        parent: [:parent]
+      }
+    end
+
     def find_epics(args)
       apply_lookahead(EpicsFinder.new(context[:current_user], args).execute)
     end

ee/app/graphql/types/epic_type.rb

@@ -32,8 +32,7 @@ module Types
     field :group, GroupType, null: false,
           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 }
+          description: 'Parent epic of the epic'
     field :author, Types::UserType, null: false,
           description: 'Author of the epic',
           resolve: -> (obj, _args, _ctx) { Gitlab::Graphql::Loaders::BatchModelLoader.new(User, obj.author_id).find }

ee/changelogs/unreleased/210553-improve-performance-of-graphql-groupepics-query-2.yml

+---
+title: Preload parent in GraphQL epics queries using Lookahead
+merge_request: 43323
+author:
+type: performance

ee/spec/requests/api/graphql/group/epics_spec.rb

@@ -17,25 +17,27 @@ RSpec.describe 'Epics through GroupQuery' do
 
   # similar to GET /groups/:id/epics
   describe 'Get list of epics from a group' do
-    let(:query) do
-      epic_node = <<~NODE
-      edges {
-        node {
-          id
-          iid
-          title
-          upvotes
-          downvotes
-          userPermissions {
-            adminEpic
+    let(:epic_node) do
+      <<~NODE
+        edges {
+          node {
+            id
+            iid
+            title
+            upvotes
+            downvotes
+            userPermissions {
+              adminEpic
+            }
           }
         }
-      }
       NODE
+    end
 
+    def query(params = {})
       graphql_query_for("group", { "fullPath" => group.full_path },
                         ['epicsEnabled',
-                         query_graphql_field("epics", {}, epic_node)]
+                         query_graphql_field("epics", params, epic_node)]
       )
     end
 
@@ -105,6 +107,42 @@ RSpec.describe 'Epics through GroupQuery' do
           end
         end
       end
+
+      context 'query performance' do
+        let!(:child_epic) { create(:epic, group: group, parent: epic2) }
+        let(:epic_node) do
+          <<~NODE
+            edges {
+              node {
+                parent {
+                  id
+                }
+              }
+            }
+          NODE
+        end
+
+        before do
+          group.reload
+          post_graphql(query, current_user: user)
+        end
+
+        it 'avoids n+1 queries when loading parent field' do
+          control_count = ActiveRecord::QueryRecorder.new(skip_cached: false) do
+            post_graphql(query, current_user: user)
+          end.count
+
+          epics_with_parent = create_list(:epic, 3, group: group) do |epic|
+            epic.update(parent: create(:epic, group: group))
+          end
+          group.reload
+
+          # Added +1 to control_count due to an existing N+1 with licenses
+          expect do
+            post_graphql(query({ iids: epics_with_parent.pluck(:iid) }), current_user: user)
+          end.not_to exceed_all_query_limit(control_count + 1)
+        end
+      end
     end
 
     context 'when error requests' do