Question

I am writing a piece of code which is going to order the list of photos based on their rating. Each photo is stored in DB, and each has such information as number of positive and negative votes. I want to order them by the formula in which I count the percentage of positive votes, and the first photo is the one with the highest percentage.

For that I used the standard IComparer interface, and wrote my own Comparer function, which compares two photos. The problem is that I do that I have to first download the list of all photos from the db. It seems like a lot of unnecessary effort which I would like to avoid. So I am wondering if it is possible to create my own SQL function which will do the comparing on the DB side, and returns to me just the photos I want? It is more efficient than comparing all the photos on the server side?

The code for my own comparer:

public class PictureComparer : IComparer<Picture>
{
    public int Compare(Picture p1, Picture p2)
    {
        double firstPictureScore = (((double)p1.PositiveVotes/(double)(p1.PositiveVotes+p1.NegativeVotes))*100);
        double secondPictureScore = (((double)p2.PositiveVotes / (double)(p2.PositiveVotes + p2.NegativeVotes)) * 100);
        if (firstPictureScore < secondPictureScore) return 1;
        if (firstPictureScore > secondPictureScore) return -1;
        return 0;
    }
}

And the code which uses the comaprer:

 var pictures = db.Pictures.Include(q => q.Tags).Include(q => q.User).ToList();
 pictures = pictures.OrderBy(q => q, new PictureComparer()).Skip(0 * 10).Take(10).ToList();

Answer 1

Remove the first call to ToList and use a lambda expression instead of defining a comparer:

var result = db.Pictures
    .Include(q => q.Tags)
    .Include(q => q.User)
    .OrderByDescending(q => 
         q.PositiveVotes + q.NegativeVotes == 0
             ? -1
             : q.PositiveVotes / (double)(q.PositiveVotes + q.NegativeVotes))
    .Skip(n * 10)
    .Take(10)
    .ToList();