LINQ：组合join，group by和select value

Question

我有一个结合了连接和组的查询，但是当我尝试加入另一个价格表时我没有得到任何结果。

我试图将所有小时数（db.WorkpaperHours）相加，按工作类型（db.WorkpaperTypes）对它们进行分组，获取该类型单位（db.Units）并获取该工作类型的价格（db.InvoicePrices）。

    var articleList = (from wp in db.Workpapers
    join wh in db.WorkpaperHours on wp.Id equals wh.WorkpaperId
    join wt in db.WorkpaperTypes on wh.WorkType equals wt.Id
    join u in db.Units on wt.Unit equals u.Id
    join ip in db.InvoicePrices on wt.Id equals ip.ArticleId //<--- this line

    where wp.InvoiceId == Id

    group wh by new
    {
        wt.Id,
        wh.WorkType,
        wt.Name,
        wt.Hour,
        wt.Vehicle,
        wt.FixedPrice,

        u.UnitName,
        UnitId = u.Id,
        Price = ip.Price
    }
    into grp
    select new InvoiceArticleList
    {
        Name = grp.Key.Name,
        NewId = grp.Key.WorkType,
        Hours = grp.Sum(p => p.Hours),
        WorkName = grp.Key.Name,

        Price = grp.Key.Price,
        FixedPrice = grp.Key.FixedPrice,


        IsHour = grp.Key.Hour,
        IsVehicle = grp.Key.Vehicle,
        UnitId = grp.Key.UnitId,
        UnitName = grp.Key.UnitName
    });

我的问题是当我试图从具有这些值的表InvoicePrices中获取价格时：

1. InvoiceId（int）（应与WorkpaperTypes.Id匹配）
2. ArticleId（int）（应与WorkpaperTypes.Id匹配）
3. 价格（十进制）（我想要获取的值）

InvoiceArticleList模型

public class InvoiceArticleList
{
    public string Name { get; set; }
    public int NewId { get; set; }
    public int Hours { get; set; }
    public string WorkName { get; set; }

    public decimal Price { get; set; }

    public bool FixedPrice { get; set; }

    public bool IsHour { get; set; }
    public bool IsVehicle { get; set; }

    public int UnitId { get; set; }
    public string UnitName { get; set; }
}