Use database query to calculate average stage position

app/models/ci/stage.rb

@@ -24,12 +24,14 @@ module Ci
       self.status = DEFAULT_STATUS if self.status.nil?
     end
 
-    before_validation do
-      next unless index.nil?
+    before_validation unless: :importing? do
+      next if index.present?
 
-      statuses.pluck(:stage_idx).tap do |indices|
-        self.index = indices.max_by { |index| indices.count(index) }
-      end
+      self.index = statuses.select(:stage_idx)
+        .where('stage_idx IS NOT NULL')
+        .group(:stage_idx)
+        .order('COUNT(*) DESC')
+        .first&.stage_idx.to_i
     end
 
     state_machine :status, initial: :created do

spec/models/ci/stage_spec.rb

@@ -92,7 +92,10 @@ describe Ci::Stage, :models do
     context 'when stage has been imported and does not have index set' do
       before do
         stage.update_column(:index, nil)
+      end
 
+      context 'when stage has statuses' do
+        before do
           create(:ci_build, :running, stage_id: stage.id, stage_idx: 10)
         end
 
@@ -104,5 +107,16 @@ describe Ci::Stage, :models do
           expect(stage.reload.index).to eq 10
         end
       end
+
+      context 'when stage does not have statuses' do
+        it 'fallbacks to zero' do
+          expect(stage.reload.index).to be_nil
+
+          stage.update_status
+
+          expect(stage.reload.index).to eq 0
+        end
+      end
+    end
   end
 end