Paging in LINQ+MVC with a randomly ordered collection

Question

I have a bunch of records that are sorted RANDOMLY, like so:

var entries = DataContext.Entries.OrderBy(e => Random());

The Random function returns a randomly-generated GUID, thereby ordering the records in a random manner. Now my problem is paging. In MVC, I have a List action for the Entry controller that lists the entries:

public class EntryController : Controller 
{
     public ActionResult List(int page)
     {
          int pageSize = 10;

          var entries = DataContext.Entries.OrderBy(e => Random()).Skip((page - 1) *      pageSize).Take(pageSize);
          ViewData["entries"] = entries;
          return View();
     }
}

My problem here is that whenever I go from one page to another page, the entries are REARRANGED ANEW. So when I go to page 1 (step 1), then go to page 2 (step 2), then back to page 1 (step 3), the entries that were shown in the step 1 are different from those shown in step 3. I absolutely need to have the records arranged randomly the first time around, but not in the subsequent look ups.

Any ideas on how best to address this problem?

@JanR i need to show an list of products which shows randomly on the pages. — Shivraj, Sep 01 '14 at 07:39
so cache a list of products? order them randomly and grab from there? if you reorder every page hit, you will never be able to paginate, you will need to rethink your design. — JanR, Sep 01 '14 at 07:51
@JanR I want to get the result as same as Justdial.com does. what they guys are doing for this? — Shivraj, Sep 01 '14 at 08:31

score 1 · Answer 1 · edited Mar 09 '16 at 06:14

1

Because each time you invoke the action the randomness occurs all over again to the whole records before paging, what you need to do is:

Order the records randomly as you want it.
Cache the result.

Fetch the cache every time the action is invoked while applying paging.

public ActionResult List(int page)
 {
      var dataSource = CacheContext["RandomRecords"];
      if(dataSource == null){
         dataSource = DataContext.Entries.OrderBy(e => Random());
      }

      int pageSize = 10;
      var entries = dataSource.Skip((page - 1) * pageSize).Take(pageSize);

      ViewData["entries"] = entries;
      return View();
 }

edited Mar 09 '16 at 06:14

bummi

27,123
14
62
101

answered Sep 01 '14 at 08:10

Samer Aburabie

248
2
8

Of course CacheContex["RandomRecords"] is just a place where you cache and only as an example here, you might have your cache any where you want so this is just for clarification purposes only. – Samer Aburabie Sep 01 '14 at 08:11
I want to get the result as same as Justdial.com does. what they guys are doing for this? – Shivraj Sep 01 '14 at 08:32

score 0 · Answer 2 · answered Mar 09 '16 at 05:21

Add a column named sortOrder, being an int. Do not forget to add a non-unique index on it or sorting on it will be suboptimal.

Put a number there, increased from record to record, not in the same order than your id or an order that would sort the records alphabetically.

Then

dataSource = DataContext.Entries.OrderBy(e => e.sortOrder);

Guids will be slower to sort on, but you can alternatively use a guid column for the task. Every now and then (once per night), you can loop through all records to put new values in the sortOrder column so people will not recognize the order.

score 0 · Answer 3 · edited Mar 09 '16 at 06:15

You can add following SQL code to get products, you only need to generate some random number and store it into the session or cache and get product again..

DECLARE @NumberOfProductsInDatabase INT
DECLARE @RandomNumber VARCHAR(10)

SET @NumberOfProductsInDatabase = (SELECT COUNT(*) FROM Product)
SET @RandomNumber = '45'
SELECT CAST(100000 *  SQRT(EXP(COS(p.SellerId + @RandomNumber))) AS INT) % @NumberOfProductsInDatabase AS RandomProductNr,
    p.Id,p.Name,pv.Price,pv.SalePrice
FROM Product p,ProductVariant pv
WHERE pv.ProductId = p.Id
ORDER BY RandomProductNr

Paging in LINQ+MVC with a randomly ordered collection

3 Answers3