Raise if updating columns in batches within a transaction

lib/gitlab/database/migration_helpers.rb

@@ -222,6 +222,12 @@ module Gitlab
       #
       # rubocop: disable Metrics/AbcSize
       def update_column_in_batches(table, column, value)
+        if transaction_open?
+          raise 'update_column_in_batches can not be run inside a transaction, ' \
+            'you can disable transactions by calling disable_ddl_transaction! ' \
+            'in the body of your migration class'
+        end
+
         table = Arel::Table.new(table)
 
         count_arel = table.project(Arel.star.count.as('count'))

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -262,7 +262,10 @@ describe Gitlab::Database::MigrationHelpers, lib: true do
   end
 
   describe '#update_column_in_batches' do
+    context 'when running outside of a transaction' do
       before do
+        expect(model).to receive(:transaction_open?).and_return(false)
+
         create_list(:empty_project, 5)
       end
 
@@ -299,6 +302,17 @@ describe Gitlab::Database::MigrationHelpers, lib: true do
       end
     end
 
+    context 'when running inside the transaction' do
+      it 'raises RuntimeError' do
+        expect(model).to receive(:transaction_open?).and_return(true)
+
+        expect do
+          model.update_column_in_batches(:projects, :star_count, Arel.sql('1+1'))
+        end.to raise_error(RuntimeError)
+      end
+    end
+  end
+
   describe '#add_column_with_default' do
     context 'outside of a transaction' do
       context 'when a column limit is not set' do