如何使用LINQ在用户之间获取最新消息? [英] How to get the last messages among users with LINQ?

问题描述

我有一个表，显示消息之间的关系.像这样:

I have a table that shows the relationship between messages. Like this:

我想从每个用户那里获得最后一条消息.因此，例如，结果将是表中的ID 91和ID 92

I want to get the last one message from each user. So, for example, result will be id 91 and id 92 from the table

var messages = await _dbContext.Messages.Include(x =>x.User).Where(x =>x.SenderId == userId | x.ReceiveId == userId).OrderByDescending(x =>x.CreatedAt).Select(x =>new Message {
  Id = x.Id,
  Content = x.Content,
  CreatedAt = x.CreatedAt,
  User = new User {
    Id = x.User.Id,
    UserName = x.User.UserName,
    Name = x.User.Name,
    LastName = x.User.LastName,
    Profil = x.User.Profil
  },
}).Distinct().ToListAsync();

我该怎么办?

推荐答案

答案是:窗口函数，EF不支持.因此，只需编写SQL并通过Dapper运行它

Answer is: Window Functions, which are not supported by EF. So just write SQL and run it via Dapper

SELECT
   s.Id,
   s.SenderId,
   s.ReceiveId,
   s.Content,
   s.CreatedAt
FROM 
(
   SELECT
      m.Id,
      m.SenderId,
      m.ReceiveId,
      m.Content,
      m.CreatedAt,
      ROW_NUMBER() OVER (PARTITION BY m.ReceiveId ORDER BY m.CreatedAt DESC) AS RN
   FROM Messages m
) s
WHERE s.RN = 1

其他解决方案只是解决方法.如果您需要LINQ来执行此任务，这是我第三次提议 linq2db.EntityFrameworkCore .>

Other solutions are just workaround. It is third time I've proposing linq2db.EntityFrameworkCore if you need LINQ for such task

var query = from m in Messages
   select new 
   {
      m.Id,
      m.SenderId,
      m.ReceiveId,
      m.Content,
      m.CreatedAt,
      RN = Sql.Ext.RowNumber().Over().PartitionBy(m.ReceiveId).OrderByDesc(m.CreatedAt).ToValue()
   }

var messageQuery = 
  from m in query
  where m.RN == 1
  select new
  {
     m.Id,
     m.SenderId,
     m.ReceiverId,
     m.Content,
     m.CreatedAt,      
  }

// switch to alternative LINQ Translator
messageQuery = messageQuery.ToLinqToDB();

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