public DynamoDBMapperFieldModel<T,R> rangeKey() {
        return this.model.rangeKey();
    }
Gets the range key field model for the specified type.
@param <R> The range key type.
@return The range key field model.
@throws DynamoDBMappingException If the range key is not present.
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/datamodeling/DynamoDBTableMapper.java