Railsドキュメント

モデル.in_batches([オプション]) do |i|
  処理内容
end

Person.where("age > 21").in_batches do |relation|
  relation.delete_all
  sleep(10) # Throttle the delete queries
end

Person.in_batches.delete_all

Person.in_batches.update_all(awesome: true)