Question

I am coding a message program something like WhatsApp.

I want to show the last message between user and his/her friend. But to do this, I am getting whole messages between them and using some query to find the last message. I think that I can re-write a better way. Because with that way, I get all message to memory. If I have a million messages, thats not a good way

    var myID = Convert.ToInt32(txtID.Text);
    var myMessages = db.MemberMessages
        .Where(m => m.MemberTo == myID || m.MemberFrom == myID)
        .Select(s => new
            {
                s.ID,
                s.Text,
                s.MemberFrom,
                s.MemberTo
            });

    List<int> messageIDs = myMessages
        .Where(s => s.MemberTo == myID).Select(s => s.MemberFrom).Distinct().ToList();

    List<int> messageIDs2 = myMessages
        .Where(s => s.MemberFrom == myID)
        .Select(s => s.MemberTo).Distinct().ToList();

    List<int> differentIDS = messageIDs2.Except(messageIDs).ToList();

    foreach (int item in differentIDS)
    {
        messageIDs.Add(item);
    }

    foreach (int userID in messageIDs)
    {
        var deger = myMessages
            .Where(m => 
                (m.MemberFrom == myID && m.MemberTo == userID) ||
                (m.MemberFrom == userID && m.MemberTo == myID))
                .OrderByDescending(m => m.ID).ThenByDescending(m => m.MemberFrom).First();

        if (deger.MemberFrom == myID)
            lstLastMessages.Items.Add(String.Format("From Me - To ID:{0}  -  Message: {1}", deger.MemberTo, deger.Text));
        else
            lstLastMessages.Items.Add(String.Format("From ID: {0} - To: Me -Message: {1}", deger.MemberFrom, deger.Text));
    }

You can see the last message between the user and his/her friends who have different ID. Is it possible to re-write it with one query? Or a shorter way?

Looking for some advice.

edit:

As you see my table, I have some record among users. For example, my ID is 1 and I want to find the last messages that I talked users before.

memberfrom 1 (its me)  memberfromTo : 2 . message: xxx 
memberfrom 2 (its me)  memberfromTo : 1 . message: yyy
memberfrom 1 (its me)  memberfromTo : 2 . message: zzzz

If you look above, I want to get the last message is zzzz. But I want to do it for each users who sent the last message.

Answer 1

Just group messages by MemberTo and MemberFrom fields, and get last message from each group ordered by CreationTime:

var query = from m in db.MemberMessages
            where m.MemberTo == id || m.MemberFrom == id
            group m by new { m.MemberTo, m.MemberFrom } into g
            select g.OrderByDescending(x => x.CreationTime).First();

This will return both last message from user and to user you are looking for (because groups will have different keys).

UPDATE

var query = from m in db.MemberMessages
            let msgTo = m.MemberTo == id
            let msgFrom = m.MemberFrom == id 
            where msgTo || msgFrom
            group m by msgTo ? m.MemberFrom : m.MemberTo into g
            select g.OrderByDescending(x => x.CreationTime).First();