protected function ensureArray($expression)
{
// Convert field names in expressions
if (is_string($expression) && substr($expression, 0, 1) === '$') {
return '$' . $this->getDocumentPersister()->prepareFieldName(substr($expression, 1));
}
// Convert PHP types to MongoDB types for everything else
return Type::convertPHPToDatabaseValue(parent::ensureArray($expression));
}