c# - 乌鸦数据库 : What's wrong with this multi-map/reduce index?

Question

我有一个跟踪网站页面访问的应用程序。这是我的模型:

public class VisitSession {
    public string SessionId { get; set; }
    public DateTime StartTime { get; set; }
    public string UniqueVisitorId { get; set; }
    public IList<PageVisit> PageVisits { get; set; }
}

当访问者访问网站时，访问 session 开始。一次访问 session 有许多页面访问。当访问者第一次访问该网站时，跟踪器将写入一个 UniqueVisitorId (GUID) cookie。因此我们能够知道访客是否是回头客。

现在我想编写一个 View ，显示每天的 TotalVisitSessions、TotalPageVisits、TotalUniqueVisitors。所以我写了这个 multi-map reduce:

public class VisitSummaryByDateIndex : AbstractMultiMapIndexCreationTask<VisitSummaryByDate>
{
    public VisitSummaryByDateIndex()
    {
        AddMap<VisitSession>(sessions => from s in sessions
                                            select new VisitSummaryByDate
                                            {
                                                Date = s.StartTime.Date,
                                                TotalVisitSessions = 1,
                                                TotalPageVisits = 0,
                                                TotalNewVisitors = s.IsNewVisit ? 1 : 0,
                                                TotalUniqueVisitors = 0,
                                                UniqueVisitorId = s.UniqueVisitorId
                                            });

        AddMap<PageVisit>(visits => from v in visits
                                    select new VisitSummaryByDate
                                    {
                                        Date = v.VisitTime.Date,
                                        TotalVisitSessions = 0,
                                        TotalPageVisits = 1,
                                        TotalNewVisitors = 0,
                                        TotalUniqueVisitors = 0,
                                        UniqueVisitorId = String.Empty
                                    });

        Reduce = results => from result in results
                            group result by result.Date into g
                            select new VisitSummaryByDate
                            {
                                Date = g.Key,
                                TotalVisitSessions = g.Sum(it => it.TotalVisitSessions),
                                TotalPageVisits = g.Sum(it => it.TotalPageVisits),
                                TotalNewVisitors = g.Sum(it => it.TotalNewVisitors),
                                TotalUniqueVisitors = g.Select(it => it.UniqueVisitorId).Where(it => it.Length > 0).Distinct().Count(),
                                UniqueVisitorId = String.Empty
                            };
    }
}

问题出在“TotalUniqueVisitors”的计算上，索引结果的TotalUniqueVisitors有时是1，有时是2。但是我查了资料，绝对不会这么少。我的 Map/Reduce 语法有问题吗？

相关帖子: Raven DB: How to create "UniqueVisitorCount by date" index

可在此处找到包含示例数据的代码: https://gist.github.com/2702071

Answer 1

Reduce 实际上对结果进行了多次处理。您的索引假设这只发生一次，并且可以访问整个结果集。

您的索引需要如下所示:

public class VisitSummaryByDateIndex : AbstractMultiMapIndexCreationTask<VisitSummaryByDate>
{
    public VisitSummaryByDateIndex()
    {
        AddMap<VisitSession>(sessions => from s in sessions
                                         select new VisitSummaryByDate
                                         {
                                             Date = s.StartTime.Date,
                                             TotalVisitSessions = 1,
                                             TotalPageVisits = 0,
                                             TotalNewVisitors = s.IsNewVisit ? 1 : 0,
                                             TotalUniqueVisitors = 1,
                                             UniqueVisitorId = new[]{s.UniqueVisitorId}
                                         });

        AddMap<PageVisit>(visits => from v in visits
                                    select new VisitSummaryByDate
                                    {
                                        Date = v.VisitTime.Date,
                                        TotalVisitSessions = 0,
                                        TotalPageVisits = 1,
                                        TotalNewVisitors = 0,
                                        TotalUniqueVisitors = 0,
                                        UniqueVisitorId = new string[0]
                                    });

        Reduce = results => from result in results
                            group result by result.Date into g
                            select new VisitSummaryByDate
                            {
                                Date = g.Key,
                                TotalVisitSessions = g.Sum(it => it.TotalVisitSessions),
                                TotalPageVisits = g.Sum(it => it.TotalPageVisits),
                                TotalNewVisitors = g.Sum(it => it.TotalNewVisitors),
                                TotalUniqueVisitors = g.Sum(it => it.TotalUniqueVisitors),,
                                UniqueVisitorId =  g.Select(x=>x.UniqueVisitorId).Distinct()
                             };
    }
}