code: public void saveIfExists(T object) throws ConditionalCheckFailedException {
          final DynamoDBSaveExpression saveExpression = new DynamoDBSaveExpression();
          for (final DynamoDBMapperFieldModel<T,Object> key : model.keys()) {
              saveExpression.withExpectedEntry(key.name(), new ExpectedAttributeValue()
                  .withExists(true).withValue(key.convert(key.get(object))));
          }
          mapper.<T>save(object, saveExpression);
      }
docs: Saves the object given into DynamoDB with the condition that the hash
  and, if applicable, the range key, already exist.
  @param object The object to update.
  @throws ConditionalCheckFailedException If the object does not exist.
  @see com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMapper#save
  @see com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBSaveExpression
  @see com.amazonaws.services.dynamodbv2.model.ExpectedAttributeValue