Reduce SQL queries needed to load open merge requests

The SQL queries and memory allocation in MergeRequests::RefreshService
is dominated by queries for Project and Route loads. On staging, the
absence of an inverse relationship caused Rails to make over 1100
extraneous SQL queries for the www-gitlab-com repository.

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/49703

2 files changed, 18 insertions, 0 deletions

diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index c8943f2d86f..85a4ebac66c 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -13,6 +13,20 @@ describe MergeRequest do
     it { is_expected.to belong_to(:merge_user).class_name("User") }
     it { is_expected.to belong_to(:assignee) }
     it { is_expected.to have_many(:merge_request_diffs) }
+
+    context 'for forks' do
+      let!(:project) { create(:project) }
+      let!(:fork) { fork_project(project) }
+      let!(:merge_request) { create(:merge_request, target_project: project, source_project: fork) }
+
+      it 'does not load another project due to inverse relationship' do
+        expect(project.merge_requests.first.target_project.object_id).to eq(project.object_id)
+      end
+
+      it 'finds the associated merge request' do
+        expect(project.merge_requests.find(merge_request.id)).to eq(merge_request)
+      end
+    end
   end
 
   describe '#squash_in_progress?' do
diff --git a/spec/models/project_spec.rb b/spec/models/project_spec.rb
index e66838edd1a..83b3f308ec3 100644
--- a/spec/models/project_spec.rb
+++ b/spec/models/project_spec.rb
@@ -88,6 +88,10 @@ describe Project do
     it { is_expected.to have_many(:project_deploy_tokens) }
     it { is_expected.to have_many(:deploy_tokens).through(:project_deploy_tokens) }
 
+    it 'has an inverse relationship with merge requests' do
+      expect(described_class.reflect_on_association(:merge_requests).has_inverse?).to eq(:target_project)
+    end
+
     context 'after initialized' do
       it "has a project_feature" do
         expect(described_class.new.project_feature).to be_present