NHibernate通过查询获取多对多

Question

我有以下数据结构：

+-----+ 1:N  +------------+ M:N  +---------+ 
|Book | ---> |BookVersion | ---> |Category | 
+-----+      +------------+      +---------+ 

Book有许多版本，每个版本都属于许多类别（简化场景）。我试图使用Futures获取带有类别的BookVersion（根据https://stackoverflow.com/a/21627753） 并试图避免笛卡尔积：

Book bookAlias = null;
BookVersion bookVersionAlias = null;
BookType bookTypeAlias = null;
Category categoryAlias = null;

using (var session = GetSession())
{
    var bookVersions =
        session.QueryOver(() => bookVersionAlias)
            .JoinAlias(() => bookVersionAlias.Categories, () => categoryAlias, JoinType.InnerJoin)
            .JoinAlias(() => bookVersionAlias.Book, () => bookAlias, JoinType.InnerJoin)
            .Where(() => categoryAlias.Type == BOOK_TYPE && bookVersionAlias.Id == bookAlias.LastVersion.Id)
            .Future<BookVersion>();

    var categories = session.QueryOver(() => categoryAlias)
        .Where(x => x.Type == BOOK_TYPE)
        .Future<Category>();

    return bookVersions.ToList();
}

结果BookVersion列表不包含类别。

如何在NHibernate 4.0中使用QueryOver获取多对多？