IndexKeysDefinitionBuilder should support defining multikey indexes on members of an array of embedded documents

public class User

{

    public Object Id { get; set; }

    public IEnumerable<Login> Logins { get; set; }

    // etc...

}

public class Login

{

    public string LoginProvider { get; set; }

    public string ProviderKey { get; set; }

    // etc...

}

var indexKeysDefinition = Builders<User>.IndexKeys

    .Ascending(user => user.Logins.Select(login => login.LoginProvider))

    .Ascending(user => user.Logins.Select(login => login.ProviderKey));

var indexKeysDefinition = Builders<User>.IndexKeys

    .Ascending("Logins.LoginProvider")

    .Ascending("Logins.ProviderKey");