Merge branch '5384_use_latest_base_pipeline_for_mr' into 'master'

Make #base_pipeline returning the latest available pipeline for MR's base commit

Closes #5384

See merge request gitlab-org/gitlab-ee!5114

app/models/merge_request.rb

@@ -1107,7 +1107,9 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def base_pipeline
-    @base_pipeline ||= project.pipelines.find_by(sha: merge_request_diff&.base_commit_sha)
+    @base_pipeline ||= project.pipelines
+      .order(id: :desc)
+      .find_by(sha: diff_base_sha)
   end
 
   def update_project_counter_caches

spec/models/merge_request_spec.rb

@@ -2404,6 +2404,26 @@ describe MergeRequest do
     end
   end
 
+  describe '#base_pipeline' do
+    let(:pipeline_arguments) do
+      {
+        project: project,
+        ref: merge_request.target_branch,
+        sha: merge_request.diff_base_sha
+      }
+    end
+
+    let(:project)       { create(:project, :public, :repository) }
+    let(:merge_request) { create(:merge_request, source_project: project) }
+
+    let!(:first_pipeline) { create(:ci_pipeline_without_jobs, pipeline_arguments) }
+    let!(:last_pipeline) { create(:ci_pipeline_without_jobs, pipeline_arguments) }
+
+    it 'returns latest pipeline' do
+      expect(merge_request.base_pipeline).to eq(last_pipeline)
+    end
+  end
+
   describe '#has_commits?' do
     before do
       allow(subject.merge_request_diff).to receive(:commits_count)