function onDeleteNullify(Model $model)
 {
     if (isset($this->through)) {
         return $this->onDeleteDelete($model);
     }
     $modelPk = Model::primaryKeyName($model);
     $queryBuilder = $model->sourceFor($model)->getBuilder();
     $queryBuilder->from(Model::tableFor($this->foreignClass))->assign($this->foreignKey, null)->equal($this->foreignKey, $model->{$modelPk});
     $source = Model::sourceFor($model);
     $source->executeStatement($queryBuilder->update(), $queryBuilder->getPdoArguments());
 }