使用.NET客户端的RavenDB Map-Reduce示例

Question

我正在寻找一个如何在RavenDB .NET客户端中实现和使用Map-Reduce的示例.

我想将它应用于特定场景:生成唯一和总访问者数量.

将存储在RavenDB中的示例文档:

public class StatisticsEntry
{
    public string Id { get; set; }
    public string UserId { get; set; }
}

我可以弄清楚如何使用Map创建标准索引,但我对如何实际使用Reduce函数,然后检索结果感到很遗憾.

遗憾的是,RavenDB网站上提供的示例并没有解释发生了什么,因此我可以通过.NET API了解如何使用它,并且示例似乎根本不使用.NET API实现这一点.

Answer 1

A map reduce index is just another way of saying "I want to do a group by", only the group by is pre-defined up front and RavenDB will process it in an efficient manner in the background so at query time you are looking up a pre-calculated result.

Consider the following as an answer as an ordinary group by (for unique users)

 var results = from doc in docs
 group doc by doc.UserId into g
 select new
 {
      g.UserId,
      g.Count()
 }

Ignoring the actual contents of the created array, we can get the total results by asking for

 results.Length

as you'd expect.

In RavenDB, you split out this function into a Map and a Reduce, and you end up with

public class UniqueVisitorsResult
{
     public string UserId { get; set; }
     public int Count { get; set; }
}

public class UniqueVisitorsIndex : AbstractIndexCreationTask<StatisticsEntry, UniqueVisitorsResult>
{
    public UniqueVisitorsIndex ()
    {
        Map = docs=> from doc in docs
                         select new 
                         { 
                             UserId = doc.UserId, 
                             Count = 1 
                         };
        Reduce = results => from result in results
                        group result by result.UserId into g
                        select new 
                        { 
                            UserId = g.Key, 
                            Count = g.Sum(x=>x.Count) 
                        };
    }
}

In essence, this is the same as the above - but you've turned it into a MapReduce function ;-)

 session.Query<StatisticEntry, UniqueVisitorsIndex>().Count();

假设Count已在LINQ提供程序中正确实现(iirc,我认为它有),将为您提供唯一访问者的总数

条目总数很简单

 session.Query<StatisticEntry>().Count();

正如您所期望的那样(无需地图/减少)

注意:此索引也可用于查看特定用户的点击次数,因为Count正在索引中计算,如果您不关心计数,则删除MapReduce的那部分并执行

public class UniqueVisitorsIndex : AbstractIndexCreationTask<StatisticsEntry>
{
    public UniqueVisitorsIndex ()
    {
        Map = docs=> from doc in docs
                     select new 
                     { 
                         UserId = doc.UserId
                     };
        Reduce = results => from result in results
                    group result by result.UserId into g
                    select new 
                    { 
                        UserId = g.Key
                    };
    }
}

Answer 2

以下是如何为唯一身份访问者构建索引:

public class Statistics_UniqueVisitors : AbstractIndexCreationTask<StatisticsEntry>
{
    public Statistics_UniqueVisitors()
    {
        Map = entries => from entry in entries
                         select new { entry.UserId, Count = 1 };
        Reduce = results => from result in results
                            group result by result.UserId into g
                            select new { UserId = g.Key, Count = g.Sum(x=>x.Count) };
    }
}

然后您可以使用以下方法查询:

var numberOfUniqueVisitors = s.Query<StatisticEntry, Statistics_UniqueVisitors>().Count();

对于访客总数,您可以使用:

var numberOfVisitors = s.Query<StatisticEntry>().Count();