Add merge requests association to pipeline

app/models/ci/pipeline.rb

@@ -17,6 +17,7 @@ module Ci
     has_many :statuses, class_name: 'CommitStatus', foreign_key: :commit_id
     has_many :builds, foreign_key: :commit_id
     has_many :trigger_requests, dependent: :destroy, foreign_key: :commit_id
+    has_many :merge_requests, foreign_key: "head_pipeline_id"
 
     has_many :pending_builds, -> { pending }, foreign_key: :commit_id, class_name: 'Ci::Build'
     has_many :retryable_builds, -> { latest.failed_or_canceled }, foreign_key: :commit_id, class_name: 'Ci::Build'

spec/workers/pipeline_metrics_worker_spec.rb

@@ -2,7 +2,7 @@ require 'spec_helper'
 
 describe PipelineMetricsWorker do
   let(:project) { create(:project, :repository) }
-  let!(:merge_request) { create(:merge_request, source_project: project, source_branch: pipeline.ref) }
+  let!(:merge_request) { create(:merge_request, source_project: project, source_branch: pipeline.ref, head_pipeline_id: pipeline.id) }
 
   let(:pipeline) do
     create(:ci_empty_pipeline,