Fix an N+1 for MRs from forks on the MR index page

app/controllers/concerns/issuable_collections.rb

@@ -165,8 +165,8 @@ module IssuableCollections
                                   [:project, :author, :assignees, :labels, :milestone, project: :namespace]
                                 when 'MergeRequest'
                                   [
-                                    :source_project, :target_project, :author, :assignee, :labels, :milestone,
-                                    head_pipeline: :project, target_project: :namespace, latest_merge_request_diff: :merge_request_diff_commits
+                                    :target_project, :author, :assignee, :labels, :milestone,
+                                    source_project: :route, head_pipeline: :project, target_project: :namespace, latest_merge_request_diff: :merge_request_diff_commits
                                   ]
                                 end
   end

changelogs/unreleased/43111-controller-projects-mergerequestscontroller-index-executes-more-than-100-sql-queries.yml

+---
+title: Reduce queries on merge requests list page for merge requests from forks
+merge_request: 18561
+author:
+type: performance

spec/support/shared_examples/issuables_list_metadata_shared_examples.rb

 shared_examples 'issuables list meta-data' do |issuable_type, action = nil|
-  before do
-    @issuable_ids = []
+  include ProjectForksHelper
+
+  def get_action(action, project)
+    if action
+      get action, author_id: project.creator.id
+    else
+      get :index, namespace_id: project.namespace, project_id: project
+    end
+  end
 
-    %w[fix improve/awesome].each do |source_branch|
-      issuable =
+  def create_issuable(issuable_type, project, source_branch:)
     if issuable_type == :issue
       create(issuable_type, project: project, author: project.creator)
     else
       create(issuable_type, source_project: project, source_branch: source_branch, author: project.creator)
     end
+  end
 
-      @issuable_ids << issuable.id
+  before do
+    @issuable_ids = %w[fix improve/awesome].map do |source_branch|
+      create_issuable(issuable_type, project, source_branch: source_branch).id
     end
   end
 
   it "creates indexed meta-data object for issuable notes and votes count" do
-    if action
-      get action, author_id: project.creator.id
-    else
-      get :index, namespace_id: project.namespace, project_id: project
-    end
+    get_action(action, project)
 
     meta_data = assigns(:issuable_meta_data)
 
@@ -29,18 +34,29 @@ shared_examples 'issuables list meta-data' do |issuable_type, action = nil|
     end
   end
 
+  it "avoids N+1 queries" do
+    control = ActiveRecord::QueryRecorder.new { get_action(action, project) }
+    issuable = create_issuable(issuable_type, project, source_branch: 'csv')
+
+    if issuable_type == :merge_request
+      issuable.update!(source_project: fork_project(project))
+    end
+
+    expect { get_action(action, project) }.not_to exceed_query_limit(control.count)
+  end
+
   describe "when given empty collection" do
     let(:project2) { create(:project, :public) }
 
     it "doesn't execute any queries with false conditions" do
-      get_action =
+      get_empty =
         if action
           proc { get action, author_id: project.creator.id }
         else
           proc { get :index, namespace_id: project2.namespace, project_id: project2 }
         end
 
-      expect(&get_action).not_to make_queries_matching(/WHERE (?:1=0|0=1)/)
+      expect(&get_empty).not_to make_queries_matching(/WHERE (?:1=0|0=1)/)
     end
   end
 end