This question has been answered by moniarde. See the first response.
class GR8BudgetUpdateFromGridProcessor extends GR8UpdateBudgetsProcessor { public function initialize() { $data = $this->getProperty('data'); if (empty($data)) return $this->modx->lexicon('invalid_data'); $data = $this->modx->fromJSON($data); $data['budgDateCreated'] = strtotime($data['budgDateCreated']); $this->primaryKeyField = 'budgId'; //<----------- HERE'S WHERE TO REDEFINE THE PRIMARY KEY if (empty($data)) return $this->modx->lexicon('invalid_data'); $this->setProperties($data); $this->unsetProperty('data'); return parent::initialize(); } } return 'GR8BudgetUpdateFromGridProcessor';