Question

I need to transform some paginated data from Linq to Entity Framework.
The data should be displayed in MVC Contrib Grid / Pager, so it needs to be paginated and the backend SQL Server should paginate the data.

So I have this code as a working base:

MyEntities db = new MyEntities();
IQueryable<HighScore> highscores = db.HighScores.OrderBy(s => s.Id);
return View(highscores.AsPagination(page.GetValueOrDefault(1), 10));

This generates a pretty paged SQL query with SELECT TOP (10) ... WHERE rownum ETC..
Great, backend SQL Server pagination, the way it should be.

So now I need to transform my entities to a little bit different model.

private HighScoreModel GetUrlForImage(string userId, int? score, bool isAnonymous)
{
  return new HighScoreModel
  {
    // transformation, left out for simplicity
  };
}

I can’t simply do:

var fixedData = from v in highscores
                select GetUrlForImage(v.UserId, v.Score, v.IsAnonymous);

because that will get me (expected):
LINQ to Entities does not recognize the method 'MvcContribTest.Models.HighScoreModel GetUrlForImage(System.String, System.Nullable1[System.Int32], Boolean)’ method, and this method cannot be translated into a store expression.`

So I need ToList() the entities in order to call my translation method, something like this:

var fixedData = from v in highscores.ToList()
                select GetUrlForImage(v.UserId,v.Score,v.IsAnonymous);

Which technically now works, but I have lost the backend SQL Server pagination because the ToList() conversion from IQueryable().

How can I do a server side pagination in this scenario?

Answer 1

What about applying pagination before the ToList then do the transformation:

var fixedData = from v in highscores.AsPagination(page.GetValueOrDefault(1), 10).ToList()
                select GetUrlForImage(v.UserId,v.Score,v.IsAnonymous);

EDIT: To use with the MVC Contrib grid you will need to wrap it up with the CustomPagination<T> class this is built in in MVC Contrib:

public ActionResult Index(int? page)
{
  var itemsPerPage = 5;
  var db = new MyEntities();
  var totalItems = db.HighScores.Count();

  IQueryable<HighScore> highscores = db.HighScores.OrderBy(highscore => highscore.ID);
  var pagedData = from highscore in highscores.AsPagination(page.GetValueOrDefault(1), itemsPerPage).ToList()
                  select highscore;

  var transformedData = new CustomPagination<HighScoreModel>(pagedData.Select(highscore => TransformData(highscore)), 
                                                             page.HasValue ? page.Value : 1, 
                                                             itemsPerPage,
                                                             totalItems);
  return View(transformedData);
}