code: public static FieldMapping parseFieldMapping(JsonNode mappingNode) {
      ValidationException.check(mappingNode.isObject(),
          "A column mapping must be a JSON record");
  
      ValidationException.check(mappingNode.has(SOURCE),
          "Partitioners must have a %s.", SOURCE);
      String source = mappingNode.get("source").asText();
  
      return parseFieldMapping(source, mappingNode);
    }
docs: Parses the FieldMapping from an annotated schema field.
  
  @param mappingNode
  The value of the "mapping" node
  @return FieldMapping