LINQ to MongoDB: .Any with a Predicate - linq

Question

I have a Collection in MongoDB of S documents. Each S has a collection of UserPermission objects, each of which have a UserId property. I want to select all the S documents that have a UserPermission with a certain UserId:
return collection.Where(s => s.UserPermissions.Any(up => up.UserId == userIdString)).ToList();
I get an error telling me that .Any with a predicate is not supported. The MongoDB docs say: "You can usually rewrite such a query by putting an equivalent where clause before the projection (in which case you can drop the projection)."
What does that mean? Any idea how I would change my query to get around this limitation?
Here's a full example. You can see I've tried 2 different queries and neither is supported:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using MongoDB.Bson;
using MongoDB.Driver;
using MongoDB.Driver.Linq;
namespace MongoSample
{
class Program
{
static void Main(string[] args)
{
App app1 = new App() { Name = "App1", Users = new List<User>()
{
new User() { UserName = "Chris" } }
};
App app2 = new App() { Name = "App2", Users = new List<User>()
{
new User() { UserName = "Chris" },
new User() { UserName = "Carlos" } }
};
MongoServer server = MongoServer.Create();
MongoDatabase database = server.GetDatabase("test");
MongoCollection appCollection = database.GetCollection("app");
appCollection.Insert(app1);
appCollection.Insert(app2);
// Throws "Any with a predicate not supported" error
//var query = appCollection.AsQueryable<App>()
// .Where(a => a.Users.Any(u => u.UserName == "Carlos"));
// Throws "Unsupported Where Clause" error.
var query = appCollection.AsQueryable<App>()
.Where(a => a.Users.Where(u => u.UserName == "Carlos").Any());
foreach (App loadedApp in query)
{
Console.WriteLine(loadedApp.ToJson());
}
Console.ReadLine();
}
}
class App
{
public string Name { get; set; }
public List<User> Users { get; set; }
}
class User
{
public string UserName { get; set; }
}
}

Answer 1

Any() without a predicate is supported, so you can do:
collection.Where(s => s.UserPermissions
.Where(up => up.UserId == userIdString).Any() )
(this is the "equivalent where clause" put before the Any)