Provide a column to grouped COUNT query

That fixes Rails warning about dangerous query method

app/models/ci/stage.rb

@@ -46,7 +46,7 @@ module Ci
       self.position = statuses.select(:stage_idx)
         .where.not(stage_idx: nil)
         .group(:stage_idx)
-        .order('COUNT(*) DESC')
+        .order('COUNT(id) DESC')
         .first&.stage_idx.to_i
     end
 

spec/models/ci/stage_spec.rb

@@ -286,6 +286,18 @@ RSpec.describe Ci::Stage, :models do
         end
       end
 
+      context 'when stage has statuses with nil idx' do
+        before do
+          create(:ci_build, :running, stage_id: stage.id, stage_idx: nil)
+          create(:ci_build, :running, stage_id: stage.id, stage_idx: 10)
+          create(:ci_build, :running, stage_id: stage.id, stage_idx: nil)
+        end
+
+        it 'sets index to a non-empty value' do
+          expect { stage.update_legacy_status }.to change { stage.reload.position }.from(nil).to(10)
+        end
+      end
+
       context 'when stage does not have statuses' do
         it 'fallbacks to zero' do
           expect(stage.reload.position).to be_nil