NHibernate Linq GroupBy多个属性不起作用

这就是我想出的代码

List<PostMonthFrequency> monthFrequencies = _postRepository
    .FindAll()
    //.ToList() //<- if included works. however not desired
    .OrderByDescending(x => x.PublishedOn)
    .GroupBy(x => new {x.PublishedOn.Year, x.PublishedOn.Month})
    .Select(post => new PostMonthFrequency { Month = new DateTime(post.Key.Year, post.Key.Month, 01), Freqency = post.Count() }).ToList();

请不要,芬德尔会把钱还给你的会话.查询()

我得到的错误是:

新建表达式

就这样。。我有其他表达式可以很好地与select一起工作,所以我不认为这是因为,更多的是与Groupby有关

提前谢谢

PS我的临时补丁

List<PostMonthFrequency> monthFrequencies = _postRepository
        .FindAll()
        .Select(x => x.PublishedOn)
        .ToList()
        .GroupBy(x => new { x.Year, x.Month })
        .Select(post => new PostMonthFrequency { Month = new DateTime(post.Key.Year, post.Key.Month, 01), Frequency = post.Count() })
        .ToList();