Question

I have a SQl query working fine, I need to convert it to LINQ and got some problems.

My working SQL query:

select d.UserID, d.Content, d.UpdateTime
from DiaryPosts as d
where d.UserID = 2
/* friends */
Union
select d.UserID, d.Content, d.UpdateTime
from DiaryPosts as d
join Friends as fr 
on d.UserID = fr.FriendID 
where fr.UserID = 2
/* following */
Union
select d.UserID, d.Content, d.UpdateTime
from DiaryPosts as d
join Followers as fl 
on d.UserID = fl.UserID
where fl.FollowerID = 2
/* ordenando por UpdateTime desc */
order by 3 desc

What I tried based on my own previous question:

var friends = (from u in db.User
                       join f in db.Friends
                       on u.ID equals f.FriendID
                       where f.UserID == userset.ID
                       orderby u.Nickname
                       select new FriendsSet { ID = u.ID, Nickname = u.Nickname, Thumbnail = u.Thumbnail }).ToList();

var followers = (from u in db.User
                         join f in db.Followers
                         on u.ID equals f.FollowerID
                         where f.UserID == userset.ID
                         orderby u.Nickname
                         select new FriendsSet { ID = u.ID, Nickname = u.Nickname }).ToList();

var diaryPosts = (from d in db.DiaryPosts
                              join e in db.EstadosDeAlma
                              on d.EstadosDeAlmaID equals e.ID
                              join u in db.User
                              on d.UserID equals u.ID
                              where d.UserID == userset.ID
                              select new DiaryPostsSet { 
                                  PostID = d.ID,
                                  EstadoDeAlmaID = e.ID,
                                  EstadoDeAlma = e.Title,
                                  Author = u.Nickname,
                                  Thumbnail = u.Thumbnail,
                                  UserID = u.ID,
                                  IsDuplicated = d.IsDuplicated,
                                  FriendID = d.FriendID,
                                  FriendName = u.Nickname,
                                  Time = d.UpdateTime,
                                  MessagesCount = d.FriendMessages.Count(m => m.DiaryPostsID == d.ID)
                              }).Take(6).ToList();

var diaryPostsUnion = diaryPosts.Union(friends).Union(followers).OrderBy(d => d.UpdateTime);

The errors I get:

'System.Collections.Generic.List<MvcWebRole1.Models.DiaryPostsSet>' does not contain a definition for 'Union' and the best extension method overload 'System.Linq.ParallelEnumerable.Union<TSource>(System.Linq.ParallelQuery<TSource>, System.Collections.Generic.IEnumerable<TSource>)' has some invalid arguments

Instance argument: cannot convert from 'System.Collections.Generic.List<MvcWebRole1.Models.DiaryPostsSet>' to 'System.Linq.ParallelQuery<MvcWebRole1.Models.FriendsSet>'

Answer 1

You’re trying to union a sequence of one type with a sequence of another type. That doesn’t make sense – unions have to be over the same types, basically.

It’s not really clear what you’re trying to do here, but I suspect you don’t want a union.

Oh, and if you want all of this to happen in the database, get rid of the ToList calls.

EDIT: Okay, it’s hard to see exactly how your LINQ queries correspond to your SQL queries, but basically your “select” clauses should be something like:

from d in db.DiaryPosts
// stuff here
select new { UserID = d.ID, d.UpdateTime, d.Content }

Use the exact same anonymous type in each query – the same property names and types, in the same order – and you should be able to use the Union method.