MongoDB C#Driver 2.0: Why can't I use a boolean method in Query?

Question

I am using the C# MongoDB driver version 2.1.0, and have the following issue: I have a static class that handles the db connection etc., where I expose properties as follows,

public static class StaticMongo {

    private readonly static MongoClient Client;
    private readonly static IMongoDatabase Database;

    public static IMongoQueryable<Game> GetGames { 
        get {
            return Database.GetCollection<Game>("Games").AsQueryable();
        } 
    }
    // ...
}

Each Game has a list of Player which contains a Name property (and more, ofc). When accessing data, eg, get all Player instances called _name from Games the following works:

var a = StaticMongo.GetHands.
    Where(n => (int)n.GameType == i).
    //Where(n => GamePassesFilter(n, true)).
    SelectMany(n => n.Players).
    Where(n => n.Name == _name).ToList();

But if I additionally want to filter them (ie, if I include the commented line), like this one,

private bool GamePassesFilter(Game _game, bool _manyAllowed) {
    var _playersInGame = _game.Players.Count();
    if (_manyAllowed && (_playersInGame == 5 ||_playersInGame == 6)) return true;
    return false;
}

I get an exception. Why? And how should this be handled corretly?

In System.ArgumentException ist eine Ausnahme vom Typ "MongoDB.Driver.dll" aufgetreten, doch wurde diese im Benutzercode nicht verarbeitet.

Zusätzliche Informationen: Unsupported filter: value(MGM8.ViewModels.PlayersViewModel).GamePassesFilter([document]).

Answer 1

User functions in IMongoQueryable.Where methods cannot be translated to FilterDefinition in the driver's PredicateTranslator .

If you are allowed to do this, you can use GamePassesFilter by calling AsEnumerable() before that:

StaticMongo.GetHands.AsEnumerable().Where(n => GamePassesFilter(n, true))...

Or, you can just write all the conditions in the query.

StaticMongo.GetHands.Where(n => n.Players.Count() == 5 || n.Players.Count() == 6 && _manyAllowed)...