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