Merge branch 'scoped-labels-ordering-fix' into 'master'

Fix selection of scoped labels

Closes #11158

See merge request gitlab-org/gitlab-ee!10993

ee/app/models/scoped_label_set.rb

@@ -28,7 +28,7 @@ class ScopedLabelSet
     by_index = label_ids_order.map.with_index { |id, idx| [id.to_i, idx] }.to_h
 
     label_ids.max do |id1, id2|
-      by_index[id1].to_i <=> by_index[id2].to_i
+      by_index.fetch(id1, -1) <=> by_index.fetch(id2, -1)
     end
   end
 

ee/spec/models/scoped_label_set_spec.rb

@@ -84,5 +84,11 @@ describe ScopedLabelSet do
 
       expect(set.last_id_by_order([kv_label1.id, kv_label3.id, kv_label2.id])).to eq(kv_label3.id)
     end
+
+    it 'returns last label present in the set ordered by custom order if there is single item' do
+      set = described_class.new('key', [kv_label1, kv_label3])
+
+      expect(set.last_id_by_order([kv_label3.id])).to eq(kv_label3.id)
+    end
   end
 end