[Rails] How to map/reduce two MongoDB collections

[Pravin Mishra]

Hi Guy's,

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 (slow) application
logic:

I have a collection 'challenge' with a 1:n relation to a collection
'tasks'. Now I'd like to receive an array of results that gives top
five heightest scored challenge.

For map I tried something like:

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


and reduce:

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

I fired this against my tasks collection:

var mr = db.tasks.mapReduce(map, reduce, { out: "results" });
But I get crucial results when querying:

db[mr.result].find();
I am using Mongoid on Rails and am completely lost with it. Can someone
point me into the right direction?

my data something  like this

/* 13 */
{
  "_id" : ObjectId("4ef1a6a454b53001a4000067"),
  "user_id" : "100002573213371",
  "title" : "social who wins",
  "description" : "social who wins",
  "updated_at" : new Date("Wed, 21 Dec 2011 14:58:04 GMT +05:30"),
  "created_at" : new Date("Wed, 21 Dec 2011 14:58:04 GMT +05:30"),
  "tasks" : [{
      "is_complete" : 0,
      "score_by" : "Check box:1 for checking off the task",
      "score" : "5",
      "name" : "task no 1",
      "_id" : ObjectId("4ef1a6a454b53001a4000068")
    }, {
      "is_complete" : 0,
      "score_by" : "Self-report number",
      "score" : "6",
      "name" : "task no 2",
      "_id" : ObjectId("4ef1a6a454b53001a4000069")
    }],
}

/* 14 */
{
  "_id" : ObjectId("4ef1a6a454b53001a400006d"),
  "canCompleteBeforeTasks" : true,
  "challenge_id" : ObjectId("4ef1a6a454b53001a4000067"),
  "created_at" : new Date("Wed, 21 Dec 2011 14:58:04 GMT +05:30"),
  "description" : "social who wins",
  "tasks" : [{
      "is_complete" : 0,
      "score_by" : "Check box:1 for checking off the task",
      "score" : "5",
      "name" : "task no 1",
      "_id" : ObjectId("4ef1a6a454b53001a400006e")
    }, {
      "is_complete" : 0,
      "score_by" : "Self-report number",
      "score" : "7",
      "name" : "task no 2",
      "_id" : ObjectId("4ef1a6a454b53001a400006f")
    }],
  "title" : "social who wins",
  "updated_at" : new Date("Wed, 21 Dec 2011 14:58:04 GMT +05:30"),
  "user_id" : "100003135115833"
}

/* 15 */
{
  "_id" : ObjectId("4ef1a6a454b53001a4000073"),
  "challenge_id" : ObjectId("4ef1a6a454b53001a4000067"),
  "created_at" : new Date("Wed, 21 Dec 2011 14:58:04 GMT +05:30"),
  "description" : "social who wins",
  "tasks" : [{
      "is_complete" : 0,
      "score_by" : "Check box:1 for checking off the task",
      "score" : "5",
      "name" : "task no 1",
      "_id" : ObjectId("4ef1a6a454b53001a4000074")
    }, {
      "is_complete" : 0,
      "score_by" : "Self-report number",
      "score" : "8",
      "name" : "task no 2",
      "_id" : ObjectId("4ef1a6a454b53001a4000075")
    }],
  "title" : "social who wins",
  "updated_at" : new Date("Wed, 21 Dec 2011 14:58:04 GMT +05:30"),
  "user_id" : "100003174704960"
}


Thx in advance.



++++++++++++++++++
Pravin Mishra

-- 
You received this message because you are subscribed to the Google Groups "Ruby 
on Rails: Talk" group.
To post to this group, send email to rubyonrails-talk@googlegroups.com.
To unsubscribe from this group, send email to 
rubyonrails-talk+unsubscr...@googlegroups.com.
For more options, visit this group at 
http://groups.google.com/group/rubyonrails-talk?hl=en.