c# Linq 两次加入同一张表

Question

使用下面的 linq 语句，我显示了一个结果列表，我加入了两个名为 AssetTransferItems 和 Memberships 的表。 Memberships 表包含一个用户列表，其中每个用户都存储为 GUID 并包含他们的用户名。

我现在想添加另一个名为 UserReceived 的列，并且像 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 UserReceived 的成员资格表中获取用户名。我添加了第二个加入：

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

但这并没有显示结果。

    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。它显示了 userReceived.Username，但没有显示 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

应该是：

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

你应该可以使用

UserReceived = userReceived.Username

而不是

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

---

编辑

从this answer 开始，以下内容应该可以工作（我删除了where 和orderby 子句以提高示例的可读性）：

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
}