code: @SuppressWarnings("unchecked")
      public <R> DynamoDBMapperFieldModel<T,R> rangeKeyIfExists() {
          return (DynamoDBMapperFieldModel<T,R>)keys.get(RANGE);
      }
docs: Gets the range key field model for the specified type.
  @param <R> The range key type.
  @return The range key field model, or null if not present.