遍历dbset以获取下n行

Question

I'm trying to find out what's a good way to continually iterate through a dbset over various function calls, looping once at the end.

Essentially I've got a bunch of Ads in the database, and I want to getNext(count) on the dbset.Ads

Here's an example

ID   Text   Other Columns...
1    Ad1    ...
2    Ad2    ...
3    Ad3    ...
4    Ad4    ...
5    Ad5    ...
6    Ad6    ...

Let's say that in my View, I determine I need 2 ads to display for User 1. I want to return Ads 1-2. User 2 then requests 3 ads. I want it to return 3-5. User 3 needs 2 ads, and the function should return ads 6 and 1, looping back to the beginning.

Here's my code that I've been working with (it's in class AdsManager ):

Ad NextAd = db.Ads.First();
public IEnumerable<Ad> getAds(count)
{
    var output = new List<Ad>();
    IEnumerable<Ad> Ads = db.Ads.OrderBy(x=>x.Id).SkipWhile(x=>x.Id != NextAd.Id);
    output.AddRange(Ads);

    //If we're at the end, handle that case
    if(output.Count != count)
    {
        NextAd = db.Ads.First();
        output.AddRange(getAds(count - output.Count));
    }
    NextAd = output[count-1];

    return output;
}

The problem is that the function call IEnumerable<Ad> Ads = db.Ads.OrderBy(x=>x.Id).SkipWhile(x=>x.Id != NextAd.Id); throws an error on AddRange(Ads) :

LINQ to Entities does not recognize the method 'System.Linq.IQueryable'1[Domain.Entities.Ad] SkipWhile[Ad](System.Linq.IQueryable'1[Domain.Entities.Ad], System.Linq.Expressions.Expression'1[System.Func`2[Domain.Entities.Ad,System.Boolean]])' method, and this method cannot be translated into a store expression.

I originally had loaded the entire dbset into a Queue, and did enqueue/dequeue, but that would not updat when a change was made to the database. I got the idea for this algorithm based on Get the next and previous sql row by Id and Name, EF?

What call should I be making to the database to get what I want?

UPDATE: Here's the working Code:

public IEnumerable<Ad> getAds(int count)
    {
        List<Ad> output = new List<Ad>();

        output.AddRange(db.Ads.OrderBy(x => x.Id).Where(x => x.Id >= NextAd.Id).Take(count + 1));

        if(output.Count != count+1)
        {
            NextAd = db.Ads.First();
            output.AddRange(db.Ads.OrderBy(x => x.Id).Where(x => x.Id >= NextAd.Id).Take(count - output.Count+1));
        }

        NextAd = output[count];
        output.RemoveAt(count);

        return output;
    }

Answer 1

SkipWhile is not supported in the EF; it can't be translated into SQL code. EF basically works on sets and not sequences (I hope that sentence makes sense).

A workaround is to simply use Where , eg:

IEnumerable<Ad> Ads = db.Ads.OrderBy(x=>x.Id).Where(x=>x.Id >= NextAd.Id);

Answer 2

Maybe you can simplify this into something like this:

Ad NextAd = db.Ads.First();

public IQueryable<Ad> getAds(count)
{
    var firstTake = db.Ads
        .OrderBy(x => x.Id)
        .Where(x => x.Id >= NextAd.Id);

    var secondTake = db.Ads
        .OrderBy(x => x.Id)
        .Take(count - result.Count());

    var result = firstTake.Concat(secondTake);

    NextAd = result.LastOrDefault()

    return result;
}

Sorry, haven't tested this, but it should work.