Fix N+1 in Epic.issues

This uses the caching_array_resolver to eliminate the N+1 for
Epic.issues.

The filtering by accessibility is removed since we have authorization to
do that for us, which is tested in the request specs.

app/graphql/resolvers/concerns/caching_array_resolver.rb

@@ -62,6 +62,7 @@ module CachingArrayResolver
         queries.in_groups_of(max_union_size, false).each do |group|
           by_id = model_class
             .from_union(tag(group), remove_duplicates: false)
+            .preload(preload) # rubocop: disable CodeReuse/ActiveRecord
             .group_by { |r| r[primary_key] }
 
           by_id.values.each do |item_group|
@@ -75,6 +76,10 @@ module CachingArrayResolver
     end
   end
 
+  def preload
+    nil
+  end
+
   # Override this to intercept the items once they are found
   def item_found(query_input, item)
   end

app/models/issue.rb

@@ -22,6 +22,7 @@ class Issue < ApplicationRecord
   include Presentable
   include IssueAvailableFeatures
   include Todoable
+  include FromUnion
 
   DueDateStruct                   = Struct.new(:title, :name).freeze
   NoDueDate                       = DueDateStruct.new('No Due Date', '0').freeze

ee/app/graphql/resolvers/epic_issues_resolver.rb

@@ -2,12 +2,26 @@
 
 module Resolvers
   class EpicIssuesResolver < BaseResolver
+    include CachingArrayResolver
+
     type Types::EpicIssueType.connection_type, null: true
 
     alias_method :epic, :object
 
-    def resolve(**args)
-      epic.issues_readable_by(context[:current_user], preload: { project: [:namespace, :project_feature] })
+    def model_class
+      ::Issue
+    end
+
+    def query_input(**args)
+      epic.id
+    end
+
+    def query_for(id)
+      ::Epic.related_issues(ids: id)
+    end
+
+    def preload
+      { project: [:namespace, :project_feature] }
     end
   end
 end

ee/spec/graphql/resolvers/epic_issues_resolver_spec.rb

@@ -3,6 +3,7 @@
 require 'spec_helper'
 
 RSpec.describe Resolvers::EpicIssuesResolver do
+  include ::Gitlab::Graphql::Laziness
   include GraphqlHelpers
 
   let_it_be(:current_user) { create(:user) }
@@ -38,17 +39,9 @@ RSpec.describe Resolvers::EpicIssuesResolver do
 
       expect(result).to eq [[issue2, issue1], [issue3, issue4]]
     end
-
-    it 'finds only epic issues that user can read' do
-      guest = create(:user)
-
-      result = epics.map { |e| resolve_epic_issues(e, user: guest).to_a }
-
-      expect(result).to eq [[issue1], []]
-    end
   end
 
   def resolve_epic_issues(object, user: current_user)
-    resolve(described_class, obj: object, ctx: { current_user: user })
+    force(resolve(described_class, obj: object, ctx: { current_user: user }))
   end
 end

ee/spec/requests/api/graphql/group/epic/epic_issues_spec.rb

@@ -113,11 +113,8 @@ RSpec.describe 'Getting issues for an epic' do
       let_it_be(:epic_issue3) { create(:epic_issue, epic: epic2, issue: issue2, relative_position: 3) }
       let(:params) { { iids: [epic.iid, epic2.iid] } }
 
-      before do
-        project.add_developer(user)
-      end
-
       it 'returns issues for each epic' do
+        project.add_developer(user)
         post_graphql(epic_query(params), current_user: user)
 
         expect(response).to have_gitlab_http_status(:success)
@@ -127,14 +124,16 @@ RSpec.describe 'Getting issues for an epic' do
       end
 
       it 'avoids N+1 queries' do
-        pending 'https://gitlab.com/gitlab-org/gitlab/-/issues/291089'
-        control_count = ActiveRecord::QueryRecorder.new do
-          post_graphql(epic_query(iid: epic.iid), current_user: user)
-        end.count
+        user_1 = create(:user, developer_projects: [project])
+        user_2 = create(:user, developer_projects: [project])
+
+        control_count = ActiveRecord::QueryRecorder.new(query_recorder_debug: true) do
+          post_graphql(epic_query(iid: epic.iid), current_user: user_1)
+        end
 
         expect do
-          post_graphql(epic_query(params), current_user: user)
-        end.not_to exceed_query_limit(control_count)
+          post_graphql(epic_query(params), current_user: user_2)
+        end.not_to exceed_query_limit(control_count).ignoring(/FROM "namespaces"/)
 
         expect(graphql_errors).to be_nil
       end