Fix N+1 in /projects REST endpoint

This commits fixes two N+1 in the /projeccts REST endpoint.

The related fields are `saml_providers` and `namespace_Settings`.

ee/changelogs/unreleased/21121-fj-fix-n-1-in-projects.yml

+---
+title: Fix N+1 in /projects endpoint
+merge_request: 57746
+author:
+type: performance

ee/lib/ee/api/entities/project.rb

@@ -11,7 +11,7 @@ module EE
 
           override :preload_relation
           def preload_relation(projects_relation, options = {})
-            super(projects_relation).with_compliance_framework_settings
+            super(projects_relation).with_compliance_framework_settings.with_group_saml_provider
           end
         end
 

ee/spec/requests/api/projects_spec.rb

@@ -90,6 +90,27 @@ RSpec.describe API::Projects do
         expect(json_response.first['id']).to eq project.id
       end
     end
+
+    context 'when there are several projects owned by groups' do
+      let_it_be(:admin) { create(:admin) }
+
+      it 'avoids N+1 queries' do
+        create(:project, :public, namespace: create(:group))
+
+        # Warming up context
+        get api('/projects', admin)
+
+        control = ActiveRecord::QueryRecorder.new do
+          get api('/projects', admin)
+        end
+
+        create_list(:project, 2, :public, namespace: create(:group))
+
+        expect do
+          get api('/projects', admin)
+        end.not_to exceed_query_limit(control.count)
+      end
+    end
   end
 
   describe 'GET /projects/:id' do

lib/api/entities/project.rb

@@ -127,7 +127,7 @@ module API
         # as `:tags` are defined as: `has_many :tags, through: :taggings`
         # N+1 is solved then by using `subject.tags.map(&:name)`
         # MR describing the solution: https://gitlab.com/gitlab-org/gitlab-foss/merge_requests/20555
-        super(projects_relation).preload(:group)
+        super(projects_relation).preload(group: :namespace_settings)
                                 .preload(:ci_cd_settings)
                                 .preload(:project_setting)
                                 .preload(:container_expiration_policy)