在 LINQ 中执行 FirstOrDefault 的替代方法

发布于 2025-01-01 00:24:27 字数 1596 浏览 3 评论 0原文

我有一个成员资格表，记录用户是否是列表的成员。当用户的成员资格发生更新时，会写入新记录，并且先前的记录保持原样，从而可以维护其成员资格的历史记录。要获取用户的会员身份，需要选择他们最近的条目。

下面是一些用户列表成员资格数据的示例。目的是找到一个按列表和用户分组但仅返回最近插入记录的行的 LINQ 表达式。

List Name, Username, Comment, ExpiresOn, Inserted
Test List, joeb, second update, 2012-03-13 16:55:03, 2012-01-31 22:28:40
Test List, joeb, first update, 2012-02-13 16:55:01, 2012-01-31 22:28:39
Test List, joeb, initial, 2012-01-13 16:55:02, 2012-01-31 22:28:38

SQL 查询说明了如何提取当前列表成员资格状态。

select ulm2.ID, ulm2.ExpiresOn, ulm2.Comment, ulm2.Inserted 
from UserListMembership as ulm1
left outer join UserListMembership ulm2 on ulm1.id = ulm2.id
group by ulm1.userlistid, ulm1.userid;

问题是如何为不使用嵌套 FirstOrDefault 调用的查询编写 LINQ 表达式，这会导致我的 MySQL 实体框架提供程序抛出“System.NotSupportedException：不支持指定的方法”。例外？

更新：

下面是我失败的 LINQ 表达式，一旦添加 FirstOrDefault 调用，它就会抛出“不支持指定的方法”。

var query = from mem in db.UserListMemberships
            where
              mem.User.UserUsernames.Any(y => y.Username.ToLower() == username.ToLower())
              && mem.UserList.Account.Subscriptions.Any(x => x.ID == subscriptionID)
              && mem.ExpiresOn > utcNow
            group mem by new { mem.UserListID, mem.UserID } into g
            select new { UserListMembership = (from mem2 in db.UserListMemberships where mem2.UserListID == g.Key.UserListID && mem2.UserID == g.Key.UserID orderby mem2.Inserted descending select mem2).FirstOrDefault() };

 return query.Select(a => a.UserListMembership).ToList();

原文

I've got a membership table that records whether a user is a member of a list. When an update to a user's membership occurs a new record is written and the previous records are left as is which allows a history of their memberships to be maintained. To get a user's membership status involves selecting their most recent entry.

An example of some user list membership data is below. The aim is to find a LINQ expression that groups by list and user but only returns the row with most recently inserted record.

List Name, Username, Comment, ExpiresOn, Inserted
Test List, joeb, second update, 2012-03-13 16:55:03, 2012-01-31 22:28:40
Test List, joeb, first update, 2012-02-13 16:55:01, 2012-01-31 22:28:39
Test List, joeb, initial, 2012-01-13 16:55:02, 2012-01-31 22:28:38

An SQL query illustrates how the current list membership status can be extracted.

select ulm2.ID, ulm2.ExpiresOn, ulm2.Comment, ulm2.Inserted 
from UserListMembership as ulm1
left outer join UserListMembership ulm2 on ulm1.id = ulm2.id
group by ulm1.userlistid, ulm1.userid;

The question is how to write a LINQ expression for the query that doesn't use a nested FirstOrDefault call which causes my MySQL entity framework provider to throw a "System.NotSupportedException: Specified method is not supported." exception?

Update:

Below is my failing LINQ expression, it throws the "Specified method is not supported" once the FirstOrDefault call is added.

var query = from mem in db.UserListMemberships
            where
              mem.User.UserUsernames.Any(y => y.Username.ToLower() == username.ToLower())
              && mem.UserList.Account.Subscriptions.Any(x => x.ID == subscriptionID)
              && mem.ExpiresOn > utcNow
            group mem by new { mem.UserListID, mem.UserID } into g
            select new { UserListMembership = (from mem2 in db.UserListMemberships where mem2.UserListID == g.Key.UserListID && mem2.UserID == g.Key.UserID orderby mem2.Inserted descending select mem2).FirstOrDefault() };

 return query.Select(a => a.UserListMembership).ToList();

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