Ensure that an imported pipeline stage can be updated

app/models/ci/stage.rb

@@ -20,10 +20,18 @@ module Ci
       validates :index, presence: true
     end
 
-    after_initialize do |stage|
+    after_initialize do
       self.status = DEFAULT_STATUS if self.status.nil?
     end
 
+    before_validation do
+      next unless index.nil?
+
+      statuses.pluck(:stage_idx).tap do |indices|
+        self.index = indices.max_by { |index| indices.count(index) }
+      end
+    end
+
     state_machine :status, initial: :created do
       event :enqueue do
         transition created: :pending

spec/models/ci/stage_spec.rb

@@ -51,7 +51,7 @@ describe Ci::Stage, :models do
     end
   end
 
-  describe 'update_status' do
+  describe '#update_status' do
     context 'when stage objects needs to be updated' do
       before do
         create(:ci_build, :success, stage_id: stage.id)
@@ -87,4 +87,21 @@ describe Ci::Stage, :models do
       end
     end
   end
+
+  describe '#index' do
+    context 'when stage has been imported and does not have index set' do
+      before do
+        create(:ci_build, :running, stage_id: stage.id, stage_idx: 10)
+        stage.update_column(:index, nil)
+      end
+
+      it 'recalculates index before updating status' do
+        expect(stage.reload.index).to be_nil
+
+        stage.update_status
+
+        expect(stage.reload.index).to eq 10
+      end
+    end
+  end
 end