public sealed class Document
{
public string Id

{ get; set; }
public List<string> Folders { get; set; }

}
public sealed class Folder
{
public string Id

{ get; set; }

}

1. var documentFolders = documentCollection.AsQueryable().Where(x => x.Id == "document1").Select(x => x.Folders).Single();
2. var folders = folderCollection.AsQueryable().Where(x => documentFolders.Contains(x.Id)).ToArray();
3. Got this ArgumentException:
Unsupported where clause: System.Collections.Generic.List`1[System.String].Contains(x.Id).

Stack trace:

at MongoDB.Driver.Linq.SelectQuery.BuildQuery(Expression expression) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Linq\Translators\SelectQuery.cs:line 862
at MongoDB.Driver.Linq.SelectQuery.BuildQuery() in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Linq\Translators\SelectQuery.cs:line 123
at MongoDB.Driver.Linq.SelectQuery.Execute() in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Linq\Translators\SelectQuery.cs:line 132
at MongoDB.Driver.Linq.MongoQueryProvider.Execute(Expression expression) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Linq\MongoQueryProvider.cs:line 147
at MongoDB.Driver.Linq.MongoQueryable`1.GetEnumerator() in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Linq\MongoQueryable.cs:line 81
at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
at DAL.MyService.MyMethod(String rId) in D:\file1.cs:line 176
at DynamicModule.ns.Wrapped_INavigationServiceRepository_364d6b61519c4b838c96d681fbc90833.<GetObjectParents_DelegateImplementation>__1(IMethodInvocation inputs, GetNextInterceptionBehaviorDelegate getNext)