title: dynamodb batch write updates existing items
body: <p>In <a href="http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_BatchWriteItem.html" rel="noreferrer">this</a> dynamodb documentation it is stated that existing items can not be updated with batch writing. However, when I try it replaces new items. How can I prevent it to update already exists one?</p>