class Node
{
    string Id { get; set; }
    List<string> ParentIds { get; set; }
}

class NodeWithChildren
{
    Node[] Children { get; set; }
}

var query = collection.Aggregate()
     .GraphLookup(
          from: collection,
          connectFromField: x => x.Id,
          connectToField: x => x.ParentIds,
          startWith: x => x.Id,
          @as: (NodeWithChildren x) => x.Children);

 
ArgumentException:  TConnectFrom must be either TConnectTo or a type that implements IEnumerable<TConnectTo>.
Parameter name: TConnectFrom

 db.Node.aggregate([
     { $graphLookup: { from: 'Node', startWith: "$_id", connectFromField: "_id", connectToField: "ParentIds", as: "Children" } }
])