check $relations is array

src/Jenssegers/Mongodb/Eloquent/Builder.php

@@ -2,6 +2,7 @@
 
 use Illuminate\Database\Eloquent\Builder as EloquentBuilder;
 use Illuminate\Database\Eloquent\Relations\Relation;
+use Illuminate\Support\Collection;
 use MongoDB\Driver\Cursor;
 use MongoDB\Model\BSONDocument;
 
@@ -167,9 +168,10 @@ class Builder extends EloquentBuilder
         $query = $hasQuery->getQuery();
 
         // Get the number of related objects for each possible parent.
+        $relations = $query->pluck($relation->getHasCompareKey());
         $relationCount = array_count_values(array_map(function ($id) {
             return (string) $id; // Convert Back ObjectIds to Strings
-        }, $query->pluck($relation->getHasCompareKey())));
+        }, is_array($relations) ? $relations : $relations->toArray()));
 
         // Remove unwanted related objects based on the operator and count.
         $relationCount = array_filter($relationCount, function ($counted) use ($count, $operator) {