code: @SuppressWarnings("unchecked")
      public <V> DynamoDBMapperFieldModel<T,V> field(final String attributeName) {
          final DynamoDBMapperFieldModel<T,V> field = (DynamoDBMapperFieldModel<T,V>)fields.get(attributeName);
          if (field == null) {
              throw new DynamoDBMappingException(
                  targetType.getSimpleName() + "[" + attributeName + "]; no mapping for attribute by name"
              );
          }
          return field;
      }
docs: Gets the field model for a given attribute.
  @param <V> The field model's value type.
  @param attributeName The attribute name.
  @return The field model.