Add a workaround for a tmp job -> stage relation

We still didn't migrate `ci_builds.stage_id`, so we can't use a
belongs_to association. We also have `ci_builds.stage` string attribute,
that we need to phase out in favor of `ci_stages.name`.

app/models/commit_status.rb

@@ -38,6 +38,15 @@ class CommitStatus < ActiveRecord::Base
   scope :retried_ordered, -> { retried.ordered.includes(project: :namespace) }
   scope :after_stage, -> (index) { where('stage_idx > ?', index) }
 
+  ##
+  # TODO, we will change this to `belongs_to :stage` when we phase out
+  # `ci_builds.stage` attribute and migrate `ci_builds.stage_id` reference in
+  # one of upcoming releases.
+  #
+  def stage_entity
+    Ci::Stage.find_by(pipeline: pipeline, name: stage)
+  end
+
   state_machine :status do
     event :enqueue do
       transition [:created, :skipped, :manual] => :pending

spec/models/commit_status_spec.rb

@@ -7,10 +7,10 @@ describe CommitStatus, :models do
     create(:ci_pipeline, project: project, sha: project.commit.id)
   end
 
-  let(:commit_status) { create_status }
+  let(:commit_status) { create_status(stage: 'test') }
 
-  def create_status(args = {})
-    create(:commit_status, args.merge(pipeline: pipeline))
+  def create_status(**opts)
+    create(:commit_status, pipeline: pipeline, **opts)
   end
 
   it { is_expected.to belong_to(:pipeline) }
@@ -408,6 +408,18 @@ describe CommitStatus, :models do
     end
   end
 
+  describe '#stage_entity' do
+    let!(:stage) do
+      create(:ci_stage_entity, pipeline: commit_status.pipeline,
+                               name: commit_status.stage)
+    end
+
+    it 'has a correct association with persisted stage' do
+      expect(commit_status.stage_entity).to eq stage
+    end
+  end
+
+
   describe '#locking_enabled?' do
     before do
       commit_status.lock_version = 100