code: public QueryBuilder mapField(String query, String field_name) throws Exception {
          JSONObject op = new JSONObject();
  
          if (query != null)
              op.put(Constants.REQUEST_QUERY, query);
  
          op.put(Constants.REQUEST_FIELD, field_name);
  
          JSONArray operations = (JSONArray) request.get(Constants.REQUEST_OPERATIONS);
          operations.put(op);
          request.remove(Constants.REQUEST_OPERATIONS);
          request.put(Constants.REQUEST_OPERATIONS, operations);
  
          return this;
      }
docs: Used to get the value from a field
  @param query
  @param field_name
  @return
  @throws Exception