Reduce pipeline serialization queries when preloaded

app/serializers/pipeline_serializer.rb

@@ -3,8 +3,11 @@ class PipelineSerializer < BaseSerializer
   entity PipelineDetailsEntity
 
   def represent(resource, opts = {})
-    if resource.is_a?(ActiveRecord::Relation)
+    if paginated? && !resource.respond_to?(:page)
+      raise Gitlab::Serializer::Pagination::InvalidResourceError
+    end
 
+    if resource.is_a?(ActiveRecord::Relation)
       resource = resource.preload([
         :stages,
         :retryable_builds,
@@ -20,7 +23,6 @@ class PipelineSerializer < BaseSerializer
     end
 
     if opts.delete(:preload)
-      resource = @paginator.paginate(resource) if paginated?
       resource = Gitlab::Ci::Pipeline::Preloader.preload!(resource)
     end
 
@@ -37,7 +39,7 @@ class PipelineSerializer < BaseSerializer
   def represent_stages(resource)
     return {} unless resource.present?
 
-    data = represent(resource, { only: [{ details: [:stages] }] })
+    data = represent(resource, { only: [{ details: [:stages] }], preload: true })
     data.dig(:details, :stages) || []
   end
 end

spec/serializers/pipeline_serializer_spec.rb

@@ -99,7 +99,8 @@ describe PipelineSerializer do
       end
     end
 
-    context 'number of queries' do
+    describe 'number of queries when preloaded' do
+      subject { serializer.represent(resource, preload: true) }
       let(:resource) { Ci::Pipeline.all }
 
       before do
@@ -120,7 +121,7 @@ describe PipelineSerializer do
         it 'verifies number of queries', :request_store do
           recorded = ActiveRecord::QueryRecorder.new { subject }
 
-          expect(recorded.count).to be_within(1).of(38)
+          expect(recorded.count).to be_within(1).of(31)
           expect(recorded.cached_count).to eq(0)
         end
       end
@@ -139,7 +140,7 @@ describe PipelineSerializer do
           # pipeline. With the same ref this check is cached but if refs are
           # different then there is an extra query per ref
           # https://gitlab.com/gitlab-org/gitlab-ce/issues/46368
-          expect(recorded.count).to be_within(1).of(45)
+          expect(recorded.count).to be_within(1).of(38)
           expect(recorded.cached_count).to eq(0)
         end
       end