code: @SuppressWarnings("unchecked")
      public <H> DynamoDBMapperFieldModel<T,H> hashKey() {
          final DynamoDBMapperFieldModel<T,H> field = (DynamoDBMapperFieldModel<T,H>)keys.get(HASH);
          if (field == null) {
              throw new DynamoDBMappingException(
                  targetType.getSimpleName() + "; no mapping for HASH key"
              );
          }
          return field;
      }
docs: Gets the hash key field model for the specified type.
  @param <H> The hash key type.
  @return The hash key field model.
  @throws DynamoDBMappingException If the hash key is not present.