按ID分组并使用Linq显示前1个

Question

Looking for a bit of advice on grouping with entity framework and linq.

So i have a table "tbl_ChatLog" that contains userID and sendToUserID etc...and with this data i'm trying to display the "Top 1" from each "SendToUserID"

So in my UI it would look like so:

• 1001 (Contains multiple but show the top 1)
• 1003 (Contains multiple but show the top 1)
• 1008 (Contains multiple but show the top 1)
• 1009 (Contains multiple but show the top 1)

The start of my code below:

public static List<Chat> getChatMessage() 
        {
            var entities = new FreeEntities();
            //My ID
            Business.User user = Business.User.getUserBySecurityToken();

             List<Chat> chatMessages = 
             (
                from cm in entities.tbl_ChatLog 
                where cm.UserID == user.uid 
                select new Chat 
                { 
                   uid = (int)cm.UserID, 
                   sendToUserID = (int)cm.SendToUserID, 
                   message = cm.Message, dateAdded = (DateTime)cm.DateAdded 
                }
             ).OrderByDescending(x => x.dateAdded).ToList();
            return chatMessages;
        }

Hoping you can help me out on this one. Grouping always seems to throw me.

Much appreciated,

Terry

Answer 1

You could use groupby :

List<Chat> chatMessages =

(from cm in entities.tbl_ChatLog 

where cm.UserID == user.uid 

orderby cm.DateAdded descending

group cm by cm.sendToUserID into grp

select new Chat
{ 

uid = grp.FirstOrDefault().UserID,

sendToUserID = grp.Key, 

message = grp.FirstOrDefault().Message,

dateAdded = grp.FirstOrDefault().DateAdded

}).ToList()

This will get you a list of your table data grouped by sendToUserID and the first entry of each group containing every property including sendToUserID .

Answer 2

The original problem was trying to select the first message from each unique "SendToUserId" and grouping by UserId and then selecting into a DTO was a nightmare but i managed to get a simple work around. Code below:

getFirstChatMessage() 
    {
        var entities = new FreeEntities();
        //User ID
        Business.User user = Business.User.getUserBySecurityToken();

        // Create new list of Chat
        List<Chat> chatList = new List<Chat>();

          var res = from c in entities.tbl_ChatLog
          where c.UserID == user.uid
          group c by c.UserID
          into groups
          select groups.OrderByDescending(p => p.DateAdded).FirstOrDefault();

        foreach (var r in res) 
        {
          chatList.Add(new Chat() 
          {
           uid = (int)r.UserID, 
           sendToUserID = (int)r.SendToUserID,
           message = (from m in entities.tbl_ChatLog where m.UserID == (int)r.SendToUserID 
           orderby r.DateAdded descending select m.Message).FirstOrDefault(),
           dateAdded = (DateTime)r.DateAdded, 
           fullName = (from b in entities.tbl_Bio where b.UserID == (int)r.SendToUserID 
           select b.FirstName + " " +      b.SurName).FirstOrDefault() 
          });
        }
        return chatList;
    }