How to make one to one mapping to respect parent/o

I have following mapping

public class FilmMap : ClassMap<Film>
{
    public FilmMap()
    {
        Id(x => x.FilmId, "film_id");
        Map(x => x.Description);
        base.HasMany<FilmActor>(x => x.FilmActor).BatchSize(100);
    }
}

public class FilmActorMap : ClassMap<FilmActor>
{
    public FilmActorMap()
    {
        Table("film_actor");
        CompositeId()
            //.KeyReference(x => x.Actor, "actor_id")
            .KeyProperty(x => x.ActorId, "actor_id")
            .KeyProperty(x => x.FilmId, "film_id");
        Map(x => x.LastUpdate, "last_update");
        References<Actor>(x => x.Actor, "actor_id"); //.Fetch.Join();
    }
}

public class ActorMap : ClassMap<Actor>
{
    public ActorMap()
    {
        Id(x => x.ActorId, "actor_id");
        Map(x => x.FirstName, "first_name");
    }
}

The code to run it

var films = session.QueryOver<Film>().Where(x => x.FilmId < 5).Future();
foreach (var film in films)
{
   foreach (var actor in film.FilmActor) //Does a batch query
   {
      Console.Write(actor.Actor.FirstName + ", "); 
      //For each actor it fetches the record from db
   }
}

When i fetch the data from Actor for each actor a single query is fired. I would like nHibernate to do a In-Query to be fired for Actor as well. Which will look like the following

SELECT actor_id, first_name
FROM actor
WHERE actor_id in (actor_id batch collected from film.FilmActor )

I do not want to do a join between the filmActor and Actor in the batch, since that is proving to be costly.

How to load the one to one map/reference to do a batch fetch

You are almost there. All the batch-size mapping you've done is correct, but BatchSize could be set not only to the Collections, but to classes as well.

public ActorMap()
{
    Id(x => x.ActorId, "actor_id");
    Map(x => x.FirstName, "first_name");
    BatchSize(100);
}

NOTE: My experience is to use it almost for every class, exactly for a reason you mentioned here... We'll gain lazy loading with more effective batching