Aggregate: FieldPath field names may not start with '$'.

[litan1106]

I got the error when I passed the $filter array into the aggregateDocumentsByFilter function. However, I didn't get error when I declared the $filter inside of the aggregateDocumentsByFilter function. (It might be the '$' issue)

** $filter = [
['$match' => ['Id' => ['$eq' => $id]]],
];

// Error
public function aggregateDocumentsByFilter($collection, $filter)
{
$cursor = $collection->aggregate($filter);

    return $cursor;

}

// No Error
public function aggregateDocumentsByFilter($collection, $id)
{
$filter = [
['$match' => ['Id' => ['$eq' => $id]]],
];

    $cursor = $collection->aggregate($filter);

    return $cursor;

}**