code: @SuppressWarnings("unchecked")
      public <R> DynamoDBMapperFieldModel<T,R> rangeKey() {
          final DynamoDBMapperFieldModel<T,R> field = (DynamoDBMapperFieldModel<T,R>)keys.get(RANGE);
          if (field == null) {
              throw new DynamoDBMappingException(
                  targetType.getSimpleName() + "; no mapping for RANGE key"
              );
          }
          return field;
      }
docs: 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.