Question

I have a page that displays two objects and then the user picks one of these. I record the preference and the combination in a MSSQL database and end up storing data like this:

UserId=1, BetterObjectId=1, WorseObjectId=2 

Now I would like to avoid showing that combination of objects (1,2 / 2,1) ever again.

So how do I generate random combinations to show the user excluding previously viewed combinations?

This seems like it should be a really straightforward question but like most programmers I’m short on sleep and coffee so your help is much appreciated 🙂

The very naive approach is something like this (and all calls to this function would have to be wrapped in a check to see if the user has already rated as many times as nCr where n is the item count and r is 2):

public List<Item> GetTwoRandomItems(int userId) {     Item i = null, i2 = null;     List<Item> r = null;      while (i == null || i2 == null)     {         r = GetTwoRandomItemsRaw();         i = r[0];         i2 = r[1];         if (GetRating(i.Id, i2.Id, userId) != null) /* Checks if viewed */         {             i = null;             i2 = null;         }     }     return r; }  private List<Item> GetTwoRandomItemsRaw() {     return Items.ToList().OrderBy(i => Guid.NewGuid()).Take(2).ToList(); } 

Edits

Using some SQL I can generate a list of all items that aren’t complete (i.e. there is a combination involving the item that the user hasn’t seen) but I don’t think is particularly useful.

I can also imagine generating every possible combination and eliminating already viewed ones before picking 2 random items but this is a another terrible solution.

A possibility (memory intensive for large n) is to generate all possible combinations and store the combinationId in the rating. Then I can just do a SELECT of all combinations WHERE combinationId IS NOT IN (SELECT combinationId FROM ratings WHERE userId=x) with some changes to reflect the symmetric relationship of combinations.

Answer 1

Table Item: ItemId Table Rating: UserId, ItemId1, ItemId2, WinnerId 

If you require that ItemId1 < ItemId2 in the Rating table, you only have to check the Rating table once.

var pair = db.Items.Join(db.Items,   i1 => i1.ItemId,   i2 => i2.ItemId,   (i1, i2) => new {i1, i2} )  //produce all pairs .Where(x => x.i1.ItemId < x.i2.ItemId) //filter diagonal to unique pairs .Where(x =>    !db.Ratings   .Where(r => r.UserId == userId     && r.ItemId1 == x.i1.ItemId     && r.ItemId2 == x.i2.ItemId)   .Any() //not any ratings for this user and pair ) .OrderBy(x => db.GetNewId()) //in-database random ordering .First();  // just give me the first one  return new List<Item>() {pair.i1, pair.i2 }; 

Here’s a blog about getting ‘random’ translated into the database.