Resolve "Search API merge_requests scope preload project features"

app/models/merge_request.rb

@@ -251,16 +251,11 @@ class MergeRequest < ApplicationRecord
   end
   scope :join_project, -> { joins(:target_project) }
   scope :references_project, -> { references(:target_project) }
-
-  PROJECT_ROUTE_AND_NAMESPACE_ROUTE = [
-    target_project: [:route, { namespace: :route }],
-    source_project: [:route, { namespace: :route }]
-  ].freeze
-
   scope :with_api_entity_associations, -> {
-    preload(:assignees, :author, :unresolved_notes, :labels, :milestone,
+    preload_routables
+      .preload(:assignees, :author, :unresolved_notes, :labels, :milestone,
                :timelogs, :latest_merge_request_diff,
-            *PROJECT_ROUTE_AND_NAMESPACE_ROUTE,
+               target_project: :project_feature,
                metrics: [:latest_closed_by, :merged_by])
   }
 
@@ -269,6 +264,10 @@ class MergeRequest < ApplicationRecord
   end
   scope :by_target_branch, ->(branch_name) { where(target_branch: branch_name) }
   scope :preload_source_project, -> { preload(:source_project) }
+  scope :preload_routables, -> do
+    preload(target_project: [:route, { namespace: :route }],
+            source_project: [:route, { namespace: :route }])
+  end
 
   scope :with_auto_merge_enabled, -> do
     with_state(:opened).where(auto_merge_enabled: true)

ee/app/models/merge_train.rb

@@ -71,8 +71,8 @@ class MergeTrain < ApplicationRecord
   scope :by_id, -> (sort = :asc) { order(id: sort) }
 
   scope :preload_api_entities, -> do
-    preload(:user, merge_request: MergeRequest::PROJECT_ROUTE_AND_NAMESPACE_ROUTE,
-                   pipeline: Ci::Pipeline::PROJECT_ROUTE_AND_NAMESPACE_ROUTE)
+    preload(:user, :merge_request, pipeline: Ci::Pipeline::PROJECT_ROUTE_AND_NAMESPACE_ROUTE)
+      .merge(MergeRequest.preload_routables)
   end
 
   class << self

ee/changelogs/unreleased/219309-fix-mr-n-plus-1.yml

+---
+title: Avoid N+1 of merge requests associations in Search
+merge_request: 36712
+author:
+type: performance

spec/services/search_service_spec.rb

@@ -374,6 +374,19 @@ RSpec.describe SearchService do
 
       subject(:result) { search_service.search_objects }
 
+      shared_examples "redaction limits N+1 queries" do |limit:|
+        it 'does not exceed the query limit' do
+          # issuing the query to remove the data loading call
+          unredacted_results.to_a
+
+          # only the calls from the redaction are left
+          query = ActiveRecord::QueryRecorder.new { result }
+
+          # these are the project authorization calls, which are not preloaded
+          expect(query.count).to be <= limit
+        end
+      end
+
       def found_blob(project)
         Gitlab::Search::FoundBlob.new(project: project)
       end
@@ -427,6 +440,12 @@ RSpec.describe SearchService do
         it 'redacts the inaccessible merge request' do
           expect(result).to contain_exactly(readable)
         end
+
+        context 'with :with_api_entity_associations' do
+          let(:unredacted_results) { ar_relation(MergeRequest.with_api_entity_associations, readable, unreadable) }
+
+          it_behaves_like "redaction limits N+1 queries", limit: 7
+        end
       end
 
       context 'project repository blobs' do
@@ -460,6 +479,10 @@ RSpec.describe SearchService do
         it 'redacts the inaccessible snippet' do
           expect(result).to contain_exactly(readable)
         end
+
+        context 'with :with_api_entity_associations' do
+          it_behaves_like "redaction limits N+1 queries", limit: 12
+        end
       end
 
       context 'personal snippets' do
@@ -471,6 +494,10 @@ RSpec.describe SearchService do
         it 'redacts the inaccessible snippet' do
           expect(result).to contain_exactly(readable)
         end
+
+        context 'with :with_api_entity_associations' do
+          it_behaves_like "redaction limits N+1 queries", limit: 3
+        end
       end
 
       context 'commits' do