How to get latest record for each group using Entity Framework and MySQL including related entity

Question

I am using Entity Framework with a MySQL Database and DbContext. I have an entity \"Message\" that has a related entity \"Sender\". (The \"Message\" also has a related entity \"

Answer 1

You don't need the SqlQuery construct to do the ordering before the grouping:

var refGroupQuery = from m in dbContext.Messages
     group m by m.receiver_id into refGroup
     let firstItem = refGroup.OrderByDescending(x => x.created_at)
                             .FirstOrDefault()
     select new MessageDTO { 
                              id = firstItem.id, 
                              content = firstItem.content,
                              sender_email = firstItem.sender.email
                           };

This does the same, but it translates the whole statement into SQL, which has two advantages

• sender is not lazy loaded for each message
• sender.email does not crash when sender is null, because in SQL there is no null object reference. The whole expression (sender.email) just returns null.