Add reviewers to MergeRequest API responses

app/models/merge_request.rb

@@ -274,7 +274,7 @@ class MergeRequest < ApplicationRecord
   scope :with_api_entity_associations, -> {
     preload_routables
       .preload(:assignees, :author, :unresolved_notes, :labels, :milestone,
-               :timelogs, :latest_merge_request_diff,
+               :timelogs, :latest_merge_request_diff, :reviewers,
                target_project: :project_feature,
                metrics: [:latest_closed_by, :merged_by])
   }

lib/api/entities/merge_request_basic.rb

@@ -27,6 +27,7 @@ module API
       expose(:downvotes)        { |merge_request, options| issuable_metadata.downvotes }
 
       expose :author, :assignees, :assignee, using: Entities::UserBasic
+      expose :reviewers, if: -> (merge_request, _) { merge_request.allows_reviewers? }, using: Entities::UserBasic
       expose :source_project_id, :target_project_id
       expose :labels do |merge_request, options|
         if options[:with_labels_details]

spec/lib/api/entities/merge_request_basic_spec.rb

@@ -40,4 +40,31 @@ RSpec.describe ::API::Entities::MergeRequestBasic do
       expect(batch.count).to be_within(3 * query.count).of(control.count)
     end
   end
+
+  context 'reviewers' do
+    context "when merge_request_reviewers FF is enabled" do
+      before do
+        stub_feature_flags(merge_request_reviewers: true)
+        merge_request.reviewers = [user]
+      end
+
+      it 'includes assigned reviewers' do
+        result = Gitlab::Json.parse(present(merge_request).to_json)
+
+        expect(result['reviewers'][0]['username']).to eq user.username
+      end
+    end
+
+    context "when merge_request_reviewers FF is disabled" do
+      before do
+        stub_feature_flags(merge_request_reviewers: false)
+      end
+
+      it 'does not include reviewers' do
+        result = Gitlab::Json.parse(present(merge_request).to_json)
+
+        expect(result.keys).not_to include('reviewers')
+      end
+    end
+  end
 end