Union order in Linq to Entities

Question

I have a problem with EDM model Union select. I have the records in db with uniqe Ids. For example id list: 1, 2, 3, 4, 5, 6, 7, 8, 9

I need to select, for example, record #6 and 2 records before #6 and 2 records past #6. In select result it should be 4,5,6,7,8

I made this next way:

public IQueryable<photos> GetNextPrev(Int64 photoid, string userlogin)
    {
        var p1 = (from m in db.photos
                 where m.id < photoid && m.userlogin == userlogin
                 orderby m.id descending
                 select m).Take(2).Skip(0);
        var p2 = (from m in db.photos
                  where m.id >= photoid && m.userlogin == userlogin
                  orderby m.id descending
                  select m).Take(3).Skip(0);
        return (p1.Union(p2));
    }

But the ordering is not like in the example...

Thanks for the help!

Solution 1

It's because of the latter Union, you cannot guarantee order with it. You want to do this on your return:

return (p1.Union(p2).OrderByDescending(m => m.id));

Update

With further understanding of the issues, I think this will take care of it:

public IQueryable<photos> GetNextPrev(Int64 photoid, string userlogin)
{
    var p1 = (from m in db.photos
             where m.id < photoid && m.userlogin == userlogin
             orderby m.id descending
             select m).Take(2).Skip(0);
    var p2 = (from m in db.photos
              where m.id >= photoid && m.userlogin == userlogin
              orderby m.id 
              select m).Take(3).Skip(0);
    return (p1.Union(p2).OrderBy(m => m.id));
}

Solution 2

You can't assume any ordering. You always need an OrderBy if you want things ordered.

return p1.Union(p2).OrderBy(p=> p.id);

Comments

[Evgeniy Labunskiy]

if i use your solution i have next one: select number 3 - result: 9,8,7,2,1; not 5,4,3,2,1.. :(

[Evgeniy Labunskiy]

thanks for your unswer but still not. now we have: I get #9 - the query show only number 9, but should 9,8,7. Maybe the link to project can help you to help me :). Thehe it is - test.wi-net.com.ua/Photos/test.

[Ralph Shillington]

@adrian, and additional problem with this proposed solution is the assumption that the ID values are contiguous. The union approach as per the OP is the surest bet.

[Justin Morgan]

You left out a descending clause on your second orderby. Was that intentional?

[Adriano Carneiro]

Yes! It should be ascending, because it will get the next 3.

[Evgeniy Labunskiy]

@adrian It works!! Thanks a lot! Like every time the solution is very simple :))