Resolve an N+1 in merge request CI variables

When building a CI pipeline, variables must be gathered at least once
per build, so it's important that the variable declarations ensure any
activerecord-dependent data is cached on the first call, to avoid
repeated SQL queries.

For this case, `present?` loads the list of assignees into memory for
subsequent calls, while `any?` issues a simple `EXISTS` query that is
repeated each time.

app/models/merge_request.rb

@@ -1290,7 +1290,7 @@ class MergeRequest < ApplicationRecord
       variables.append(key: 'CI_MERGE_REQUEST_PROJECT_URL', value: project.web_url)
       variables.append(key: 'CI_MERGE_REQUEST_TARGET_BRANCH_NAME', value: target_branch.to_s)
       variables.append(key: 'CI_MERGE_REQUEST_TITLE', value: title)
-      variables.append(key: 'CI_MERGE_REQUEST_ASSIGNEES', value: assignee_username_list) if assignees.any?
+      variables.append(key: 'CI_MERGE_REQUEST_ASSIGNEES', value: assignee_username_list) if assignees.present?
       variables.append(key: 'CI_MERGE_REQUEST_MILESTONE', value: milestone.title) if milestone
       variables.append(key: 'CI_MERGE_REQUEST_LABELS', value: label_names.join(',')) if labels.present?
       variables.concat(source_project_variables)

changelogs/unreleased/198694-speed-up-new-merge-request-worker.yml

+---
+title: Resolve an N+1 in merge request CI variables
+merge_request: 28688
+author:
+type: performance

spec/models/merge_request_spec.rb

@@ -3701,4 +3701,18 @@ describe MergeRequest do
       end
     end
   end
+
+  describe '#predefined_variables' do
+    let(:merge_request) { create(:merge_request) }
+
+    it 'caches all SQL-sourced data on the first call' do
+      control = ActiveRecord::QueryRecorder.new { merge_request.predefined_variables }.count
+
+      expect(control).to be > 0
+
+      count = ActiveRecord::QueryRecorder.new { merge_request.predefined_variables }.count
+
+      expect(count).to eq(0)
+    end
+  end
 end