code: public Fetch field(String field, Fetch fetch) {
      attrFetchMap.put(field, fetch);
      return this;
    }
docs: Updates this fetch, adding a single field. If the field is a reference, the reference will be
  fetched with the Fetch that is provided.
  
  @param field the name of the field to fetch
  @param fetch the fetch to use for this field, if the field is a reference
  @return this Fetch, updated