Question

I’m building an auction site and the user can bid on the same item more than once (obviously). In the user’s dashboard, a user can view his bids. When the user bids on the same item more than once, I want only one entry with the highest bid value to show up. My current code shows an entry for each bid. I tried a few things but I couldn’t figure it out. Here’s what I’ve got:

public class Bid
{
    public int Id { get; set; }
    public double Amount { get; set; }
    public DateTime Date { get; set; }
    public virtual Item Item { get; set; }
    public virtual User User { get; set; }
}

    protected override List<ItemForUserBids> ResolveCore(User source)
    {
        var items = new List<ItemForUserBids>();
        var userBids = source.Bids;
        foreach (var bid in userBids)
        {
            var item = bid.Item;
            var c = new ItemForUserBids
                        {
                            BidValue = bid.Amount,
                            BidId = bid.Id,
                            Description = item.Description,
                            Id = item.Id,
                            ItemThumb = item.MainImageLink(),
                            Status = _itemsService.GetBiddingStatus(item, source),
                            TimeLeft = item.EndDate.TimeLeft(),
                            Title = item.Title
                        };
            items.Add(c);
        }
        return items;
    }

I tried to get Distinct bids based on the Item.Id but that did not work. Now I’m thinking maybe I could use the Date property of the Bid entity somehow to get to the result I want but my head stopped thinking.

Any suggestions?

UPDATE:

I got it to work using a dictionary and using OrderBy() and Max() like many suggested. But I think the latter could be further improved.

Implementation using a dictionary (works):

    var userBids = new Dictionary<string, Bid>();

    foreach (var bid in allUserBids)
    {
        var key = bid.Item.Id.ToString();
        if(userBids.ContainsKey(key))
        {
            if (userBids[key].Amount < bid.Amount)
                userBids[key] = bid;
        }
        userBids[key] = bid;
    }

Attempt using the other method (works):

    var highestBids =
        source.Bids.Where(x => x.Date > DateTime.Now.AddYears(-1))
                    .GroupBy(x => x.Item.Id,
                                            (itemId, bids) =>
                                            new
                                                {
                                                    ItemId = itemId,
                                                    MaxBid = bids.Max(x => x.Amount)
                                                }).ToList();
    var userBids = new List<Bid>();
    foreach (var bid in source.Bids)
    {
        for(var i = 0; i < highestBids.Count; i++)
        {
            var curr = highestBids[i];
            if (bid.Item.Id.Equals(curr.ItemId) && bid.Amount.Equals(curr.MaxBid)) {
                userBids.Add(bid);
                highestBids.Remove(curr);
            }
        }
    }

How do I get rid of those loops? And maybe have it all in one chained statement?

Answer 1

The comments posted so far should be a good indication that you should look into re-architecting this a little, but the immediate code solution involves using System.Linq to chain together a GroupBy, Max, and a Select.