Runtime error while using a linq query

Question

I am able to write this code for a Web Api. In this code i have written tow queries and combined both in a way that comments and follower data should be merged (not combined) with respect to time based on ctime and startedfollowing. If a user has a new comment, the comment should come first and if the follower is first, the follower data should come first.

 public IQueryable<Object> GetCommentsandFollowActivityCommnets()
    {

        var combo1 = from c in db.comments
                    join p in db.picturedetails on c.targetpictureid equals p.idpictures
                    join u in db.users on c.iduser equals u.iduser
                    select new TCommentDTO
                    {

                        idcomments=c.idcomments,
                        comment1 = c.comment1,
                        targetpictureid = c.targetpictureid,
                        ctime = c.ctime,
                        iduofpic=p.iduser,
                        iduofcommentor=c.iduser,
                        profilepicofcommentor=u.profilepic,
                        usernameofcommentor=u.username,
                        picFilename=p.picFilename,
                        picTitle=p.picTitle

                    };

      var combo2=  from f in db.followers
                    join u in db.users on f.iduser equals u.iduser
                    select new TfollowerDTO
                    {    
                        idfollowers=f.idfollowers,
                        iduser=f.iduser,
                        targetiduser=f.targetiduser,
                        startedfollowing=f.startedfollowing,
                        unoffollower=u.username,
                        ppoffollower=u.profilepic,
                        status=u.status


                    };
          var result1 = from c in combo1
          select new UserTimeLineDTO
          { SortKey = c.ctime, Member =c};

          var result2 = from c in combo2
          select new UserTimeLineDTO{ SortKey = c.startedfollowing, Member = c };

            var result = result1.Concat(result2).OrderBy(x =>x.SortKey).Select(x => x.Member);

            return result; 

    }

The code is not giving any compile time error. It runs fine in compiler, but on run time i am getting an exception:

DbUnionAllExpression requires arguments with compatible collection ResultTypes.

How to remove this exception?

Wiktor Zychla · Accepted Answer · 2013-10-31 15:07:29Z

1

As a workaround, I would try to evaluate the last expression in memory:

      var result1 = (from c in combo1
      select new UserTimeLineDTO
      { SortKey = c.ctime, Member =c}).ToList();

      var result2 = (from c in combo2
      select new UserTimeLineDTO{ SortKey = c.startedfollowing, Member = c }).ToList();

      var result = result1.Concat(result2).OrderBy(x =>x.SortKey).Select(x => x.Member);

The union should now succeed, as well as ordering and the final projection.

answered Oct 31, 2013 at 15:07

Wiktor Zychla

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1 Comment

Wiktor Zychla Over a year ago

.ToList forces the evaluation and materializes objects in memory. This way the Concat is evaluated in memory rather than at the database side.

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