1 files changed, 43 insertions, 0 deletions
diff --git a/spec/lib/api/entities/merge_request_basic_spec.rb b/spec/lib/api/entities/merge_request_basic_spec.rb
new file mode 100644
index 00000000000..715fcf4bcdb
--- /dev/null
+++ b/spec/lib/api/entities/merge_request_basic_spec.rb
@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe ::API::Entities::MergeRequestBasic do
+  let_it_be(:user) { create(:user) }
+  let_it_be(:project) { create(:project, :public) }
+  let_it_be(:merge_request) { create(:merge_request) }
+  let_it_be(:labels) { create_list(:label, 3) }
+  let_it_be(:merge_requests) { create_list(:labeled_merge_request, 10, :unique_branches, :with_diffs, labels: labels) }
+
+  # This mimics the behavior of the `Grape::Entity` serializer
+  def present(obj)
+    described_class.new(obj).presented
+  end
+
+  context "with :with_api_entity_associations scope" do
+    let(:scope) { MergeRequest.with_api_entity_associations }
+
+    it "avoids N+1 queries" do
+      query = scope.find(merge_request.id)
+
+      control = ActiveRecord::QueryRecorder.new do
+        present(query).to_json
+      end
+
+      # stub the `head_commit_sha` as it will trigger a
+      # backward compatibility query that is out-of-scope
+      # for this test whenever it is `nil`
+      allow_any_instance_of(MergeRequestDiff).to receive(:head_commit_sha).and_return(Gitlab::Git::BLANK_SHA)
+
+      query = scope.all
+      batch = ActiveRecord::QueryRecorder.new do
+        entities = query.map(&method(:present))
+
+        entities.to_json
+      end
+
+      # The current threshold is 3 query per entity maximum.
+      expect(batch.count).to be_within(3 * query.count).of(control.count)
+    end
+  end
+end