Question

I am new to map / reduce and trying to figure out a way to collect the following data using map / reduce instead doing it my my (slow) application logic:

我有一个收集项目,与收集任务有1:n的关系。现在,我要收到一系列成果,给我留下第一个是项目,任务最重,最后一个是项目,任务最少。

甚至更完善的一套轮 has也告诉我每个项目有多少任务(假设项目名称是独一无二的):

[project_1: 23, project_2: 42, project_3: 82]

关于地图,我尝试了这样的东西:

map = function () {
  emit(this.project_id, { count:1 });
}

减少:

reduce = function (key, values) {
  var sum = 0;
  values.forEach(function(doc){ sum += 1; });
  return { count:sum };
}

我对我的任务进行了评价:

var mr = db.tasks.mapReduce(map, reduce, { out: "results" });

But I get crucial results when querying:

db[mr.result].find();

我在铁路上使用Mongoid,并完全失去。谁能把我引向正确的方向?

Thx in advance. Felix

Answer 1

从总体上看,我至少发现一个问题: 削减职能中的总结步骤应当

  values.forEach(function(doc){ sum += doc.count ; });

由于这一职能可能减少以前削减步骤所形成的价值,因此,其价值和价值可乘;

That s a common oversight, mentioned here: http://www.mongodb.org/display/DOCS/Troubleshooting+MapReduce

希望帮助!

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