C＃Linq两次联接同一表

Question

使用下面的linq语句，我显示了一个结果列表，该列表连接了两个名为AssetTransferItems和Memberships的表。 成员资格表包含一个用户列表，其中每个用户均以GUID形式存储，并包含其用户名。

现在，我想添加另一个名为UserReceived的列，并像UserAdded一样显示用户名而不是GUID。 UserReceived = txboxItems.UserReceived当前显示GUID，我正在尝试获取用户名。

我不确定如何修改Linq语句以从成员资格表中获取UserReceived的用户名。 我添加了第二个联接：

join userReceived in Memberships on txboxItems.UserReceived equals userReceived.UserId

但这没有显示结果。

    var query = (from txboxItems in AssetTransferItems
                                        join user in Memberships on txboxItems.UserAdded equals user.UserId
                                        join userReceived in Memberships on txboxItems.UserReceived equals userReceived.UserId

                                     where txboxItems.TransferBoxID == BoxId && txboxItems.Deleted == false
                                    orderby txboxItems.TicketID descending
                                     select new
                                    {
                                        Description = txboxItems.Description.ToString(),
                                        DateAdded = (DateTime?) txboxItems.DateAdded.Value,
                                        UserAdded = user.Username,
                                        DateReceived = (DateTime?) txboxItems.DateReceived.Value,
                                        UserReceived = userReceived.Username,
                                    });

编辑：我更新了linq语句以反映我现在所拥有的。 它显示userReceived.Username，但不显示UserReceived为null的所有其他结果。

谢谢

Answer 1

join user in Memberships on txboxItems.UserAdded equals user.UserId
join userReceived in Memberships on txboxItems.UserAdded equals userReceived.UserId

应该：

join user in Memberships on txboxItems.UserAdded equals user.UserId
join userReceived in Memberships on txboxItems.UserReceived equals userReceived.UserId
// ------------------------------------------------^^^^^^^^

然后，您应该可以使用

UserReceived = userReceived.Username

代替

UserReceived = txboxItems.UserReceived
// ------------^^^^^^^^^^^^^^^^^^^^^^^

---

编辑

从这个答案中 ，以下应该起作用（为了示例的可读性，我删除了where和orderby子句）：

from txboxItems in AssetTransferItems
from user in Memberships
    .Where(u => u.UserId == txboxItems.UserAdded).DefaultIfEmpty()
from userReceived in Memberships
    .Where(u => u.UserId == txboxItems.UserReceived).DefaultIfEmpty()
select new
{
     Description = txboxItems.Description.ToString(),
     DateAdded = (DateTime?) txboxItems.DateAdded.Value,
     UserAdded = user?.Username,
     DateReceived = (DateTime?) txboxItems.DateReceived.Value,
     UserReceived = userReceived?.Username
}