Question

I have a news table and I would like to implement custom ordering. I have done this before via a positional mapping table which has newsIds and a position.

I then LEFT OUTER JOIN the position table ON news.newsId = position.itemId with a select case statement

CASE WHEN [position] IS NULL THEN 9999 ELSE [position] END

and order by position asc, articleDate desc.

Now I am trying to do the same with Linq to Entities. I have set up my tables with a PK, FK relationship so that my News object has an Entity Collection of positions.

Now comes the bit I can’t work out. How to implement the LEFT OUTER JOIN.

I have so far:

 var query = SelectMany (n => n.Positions, (n, s) => new { n, s })
                    .OrderBy(x => x.s.position)
                    .ThenByDescending(x => x.n.articleDate)
                    .Select(x => x.n);

This kinda works. However this uses a INNER JOIN so not what I am after.

I had another idea:

 ret = ret.OrderBy(n => n.Positions.Select(s => s.position));

However I get the error DbSortClause expressions must have a type that is order comparable.

I also tried

ret = ret.GroupJoin(tse.Positions, n => n.id, s => s.itemId, (n, s) => new { n, s })
                    .OrderBy(x => x.s.Select(z => z.position))
                    .ThenByDescending(x => x.n.articleDate)
                    .Select(x => x.n);

but I get the same error!

If anyone can help me out, it would be much appreciated!

UPDATE:
So after some playing about I have managed to get it working.

ret = ret.GroupJoin(entity.Positions, n => n.id, s => s.itemId, (n, s) => new { n, s })
                    .SelectMany(x => x.n.Positions.DefaultIfEmpty(), (n, s) => new { n, s })
                    .OrderBy(x => x.s.position)
                    .ThenByDescending(x => x.n.n.articleDate)
                    .Select(x => x.n.n);

However, this is still not completely right. I have no way of using only a specific positionid or articleType.

If I have news id 1 and a review id but defined in the positions table, currently (I think) the linq query would select both?

If I try and use a where clause, it’s basically the same as inner joining. What I need is to try and use a case in the select like I would do in straight SQL:

CASE WHEN [position] IS NULL OR shuffleId != 1 THEN 9999 ELSE [position] END

I may have to completely rethink how do this. Don’t suppose anyone has any suggestions of alternative methods?

Answer 1

How about:

var query =
   from n in news
   let p = n.Positions.Select(p=>p.Position).FirstOrDefault() ?? 9999
   orderby p, n.ArticleDate
   select n;

Ps. the above assumes there is only 1 position entry for each news … just seems weird if that were not the case.

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Solution posted by OP in the comment below:

ret = ret
      .GroupJoin(tse.Positions, n => n.id, s => s.itemId, (n, s) => new { n, s }) 
      .SelectMany(x => x.s.DefaultIfEmpty(), (n, s) => new { n, s }) 
      .OrderBy(x => x.s.position == null || x.s.positionId != positionId ? 9999 : x.s.position) 
      .ThenByDescending(x => x.n.n.articleDate) 
      .Select(x => x.n.n);