Abstract persisted/legacy stages in pipeline model

app/controllers/projects/pipelines_controller.rb

 class Projects::PipelinesController < Projects::ApplicationController
-  # before_action :whitelist_query_limiting, only: [:create, :retry]
+  before_action :whitelist_query_limiting, only: [:create, :retry] # TODO?
   before_action :pipeline, except: [:index, :new, :create, :charts]
   before_action :commit, only: [:show, :builds, :failures]
   before_action :authorize_read_pipeline!

app/models/ci/pipeline.rb

@@ -249,6 +249,16 @@ module Ci
       stage unless stage.statuses_count.zero?
     end
+    ##
+    # TODO consider switching to persisted stages only in pipelines table
+    # (not necessairly in the show pipeline page because of #23257.
+    # Hide this behind two feature flags - enabled / disabled and only
+    # gitlab-ce / everywhere.
+    #
+    def stages
+      super
+    end
+
     def legacy_stages
       # TODO, this needs refactoring, see gitlab-ce#26481.

app/serializers/pipeline_details_entity.rb

 class PipelineDetailsEntity < PipelineEntity
   expose :details do
-    ##
-    # TODO consider switching to persisted stages only in pipelines table
-    # (not necessairly in the show pipeline page because of #23257.
-    # Hide this behind two feature flags - enabled / disabled and only
-    # gitlab-ce / everywhere.
-    expose :stages, as: :stages, using: StageEntity
+    expose :stages, using: StageEntity
     expose :artifacts, using: BuildArtifactEntity
     expose :manual_actions, using: BuildActionEntity
   end

spec/controllers/projects/pipelines_controller_spec.rb

@@ -17,8 +17,7 @@ describe Projects::PipelinesController do
   describe 'GET index.json' do
     before do
-      %w(pending running running success canceled)
-        .each_with_index do |status, index|
+      %w(pending running success failed).each_with_index do |status, index|
           create_pipeline(status, project.commit("HEAD~#{index}"))
         end
     end
@@ -32,11 +31,13 @@ describe Projects::PipelinesController do
       expect(response).to match_response_schema('pipeline')
       expect(json_response).to include('pipelines')
-      expect(json_response['pipelines'].count).to eq 5
-      expect(json_response['count']['all']).to eq '5'
-      expect(json_response['count']['running']).to eq '2'
+      expect(json_response['pipelines'].count).to eq 4
+      expect(json_response['count']['all']).to eq '4'
+      expect(json_response['count']['running']).to eq '1'
       expect(json_response['count']['pending']).to eq '1'
       expect(json_response['count']['finished']).to eq '2'
+      puts queries.log
+      puts "Queries count: #{queries.count}"
       expect(queries.count).to be < 32
     end