Introduce latest_status and add a few tests

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7333#note_20003268

app/models/ci/pipeline.rb

@@ -98,6 +98,10 @@ module Ci
       end.where(id: max_id.group(:ref, :sha))
     end
 
+    def self.latest_status(ref = nil)
+      latest(ref).status
+    end
+
     def self.latest_successful_for(ref)
       success.latest(ref).first
     end

app/models/commit.rb

@@ -230,7 +230,7 @@ class Commit
 
     return @statuses[ref] if @statuses.key?(ref)
 
-    @statuses[ref] = pipelines.latest(ref).status
+    @statuses[ref] = pipelines.latest_status(ref)
   end
 
   def revert_branch_name

app/services/ci/image_for_build_service.rb

@@ -5,7 +5,7 @@ module Ci
       sha = opts[:sha] || ref_sha(project, ref)
       pipelines = project.pipelines.where(sha: sha)
 
-      image_name = image_for_status(pipelines.latest(ref).status)
+      image_name = image_for_status(pipelines.latest_status(ref))
       image_path = Rails.root.join('public/ci', image_name)
 
       OpenStruct.new(path: image_path, name: image_name)

spec/models/ci/pipeline_spec.rb

@@ -381,6 +381,63 @@ describe Ci::Pipeline, models: true do
     end
   end
 
+  shared_context 'with some empty pipelines' do
+    before do
+      create_pipeline(:canceled, 'ref', 'A')
+      create_pipeline(:success, 'ref', 'A')
+      create_pipeline(:failed, 'ref', 'B')
+      create_pipeline(:skipped, 'feature', 'C')
+    end
+
+    def create_pipeline(status, ref, sha)
+      create(:ci_empty_pipeline, status: status, ref: ref, sha: sha)
+    end
+  end
+
+  describe '.latest' do
+    include_context 'with some empty pipelines'
+
+    context 'when no ref is specified' do
+      let(:pipelines) { Ci::Pipeline.latest.all }
+
+      it 'returns the latest pipeline for the same ref and different sha' do
+        expect(pipelines.map(&:sha)).to contain_exactly('A', 'B', 'C')
+        expect(pipelines.map(&:status)).
+          to contain_exactly('success', 'failed', 'skipped')
+      end
+    end
+
+    context 'when ref is specified' do
+      let(:pipelines) { Ci::Pipeline.latest('ref').all }
+
+      it 'returns the latest pipeline for ref and different sha' do
+        expect(pipelines.map(&:sha)).to contain_exactly('A', 'B')
+        expect(pipelines.map(&:status)).
+          to contain_exactly('success', 'failed')
+      end
+    end
+  end
+
+  describe '.latest_status' do
+    include_context 'with some empty pipelines'
+
+    context 'when no ref is specified' do
+      let(:latest_status) { Ci::Pipeline.latest_status }
+
+      it 'returns the latest status for the same ref and different sha' do
+        expect(latest_status).to eq(Ci::Pipeline.latest.status)
+      end
+    end
+
+    context 'when ref is specified' do
+      let(:latest_status) { Ci::Pipeline.latest_status('ref') }
+
+      it 'returns the latest status for ref and different sha' do
+        expect(latest_status).to eq(Ci::Pipeline.latest_status('ref'))
+      end
+    end
+  end
+
   describe '#status' do
     let!(:build) { create(:ci_build, :created, pipeline: pipeline, name: 'test') }
 

spec/models/commit_spec.rb

@@ -224,7 +224,7 @@ eos
       end
 
       it 'gives compound status from latest pipelines' do
-        expect(commit.status).to eq(Ci::Pipeline.latest.status)
+        expect(commit.status).to eq(Ci::Pipeline.latest_status)
       end
     end
 
@@ -251,7 +251,7 @@ eos
       end
 
       it 'gives compound status from latest pipelines if ref is nil' do
-        expect(commit.status(nil)).to eq(Ci::Pipeline.latest.status)
+        expect(commit.status(nil)).to eq(Ci::Pipeline.latest_status)
       end
     end
   end