MongoDB MapReduce on deep nested documents issue

Question

For my Rails+MongoId application i need to aggregate data for statistical purposes. My model is rapresenting a general Web application that has_many versions (aka: bundles) and relative users activations:

App = {
  "_id" : ObjectId("4ff2e2eab528571384000eb4"),
  "name" : "my app",
  "category_id": "4ff2e2eab528571384000cc0",
  "bundles": [{
    "_id" : ObjectId("4ff2e2eab528571384000dca"),
    "activations" : [{
      "user" : "user_0",
      "_id" : ObjectId("4ff2e2eab528571384000dd0")
     },
     ... 
  },
  ...
  ]
}

Now, what i have to do is to return the number of ACTIVATIONS per application CATEGORY_ID, alà:

results = [
  {
    "_id": "4ff2e2eab528571384000cc0",
    "value": 243
  },
  ...
]

I can easily obtain the total number of BUNDLES per CATEGORY_ID with this mapreduce function:

map = function() { 
  for (var bundle in this.bundles) { 
    emit(this.category_id, 1); 
  }
};
reduce = function(key, values) { 
  var sum = 0; 
  values.forEach(function(value) { 
    sum += value; 
  }); 
  return sum; 
};

But, when i try to enter one level deeper, i do obtain no results at all:

map = function() { 
  for (var bundle in this.bundles) { 
    for (var activation in bundle.activations) {
      emit(this.category_id, 1); 
    }
  }
};
reduce = function(key, values) {
  var sum = 0; 
  values.forEach(function(value) { 
    sum += value;
  });
  return sum;
};

Any idea is greatly appreciated. Best regards

Mike Costa

score 1 · Answer 1 · answered Jul 04 '12 at 13:19

Thanks for the tips NOMOA,

i'm able solve my problem by modifying the map function as the following:

map = function() { 
  var len = this.bundles.length; 
  for (var i = 0; i < len; i++) {
    if (this.bundles[i].activations !== undefined) { 
      emit(this.category_id, this.bundles[i].activations.length);
    } 
  }
};

I had to remove the for..in contruct in favor of a more traditional for..i++, add an "undefined" check (as you suggest) and it works perfectly (i also avoid the double loop)!

Best regards M. Costa

score 0 · Answer 2 · answered Jul 03 '12 at 14:23

0

Are you sure that your map reduce function run correctly?

I guess you should check if bundle.activations is non null. Check within server logs for any errors. Also with server logs you can view debug print statements.

answered Jul 03 '12 at 14:23

nomoa

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MongoDB MapReduce on deep nested documents issue

2 Answers2