"Map Reduce" reduce function finds value undefined

Question

I have the following collection:

{
  "_id" : ObjectId("51f1fcc08188d3117c6da351"),
  "cust_id" : "abc123",
  "ord_date" : ISODate("2012-10-03T18:30:00Z"),
  "status" : "A",
  "price" : 25,
  "items" : [{
      "sku" : "ggg",
      "qty" : 7,
      "price" : 2.5
    }, {
      "sku" : "ppp",
      "qty" : 5,
      "price" : 2.5
    }]
}

My map function is:

 var map=function(){emit(this._id,this);}

For debugging purpose I overide the emit method as follows:

var emit = function (key,value){
  print("emit");
  print("key: " + key + "value: " + tojson(value));
  reduceFunc2(key, toJson(value));
}

and the reduce function as follows:

var reduceFunc2 = function reduce(key,values){
  var val = values;
  print("val",val);
  var items = [];
  val.items.some(function (entry){
    print("entry is:::"+entry);
    if (entry.qty>5 && entry.sku=='ggg'){
      items.push(entry)
    }
  });
  val.items = items;
  return val;
}

But when I apply map as:

var myDoc = db.orders.findOne({
  _id: ObjectId("51f1fcc08188d3117c6da351")
});
map.apply(myDoc);

I get the following error:

emit key: 51f1fcc08188d3117c6da351 value:
{
  "_id":" ObjectId(\"51f1fcc08188d3117c6da351\")",
  "cust_id":"abc123",
  "ord_date":" ISODate(\"2012-10-03T18:30:00Z\")",
  "status":"A",
  "price":25,
  "items":[
    {
       "sku":"ggg",
       "qty":7,
       "price":2.5
    },
    {
       "sku":"ppp",
       "qty":5,
       "price":2.5
    }
  ]
}

value:: undefined
Tue Jul 30 12:49:22.920 JavaScript execution failed: TypeError: Cannot call method 'some' of undefined

you can find that their is an items field in the value as printed which is of array kind, even then it is throwing error cannot call some on undefined, if someone can tell where i am going wrong.

Answer 1

You have an error in your reduceFunc2 function:

var reduceFunc2 = function reduce(key,values){
  var val = values[0]; //values is an array!!!
  // ...
}

Reduce function meant to reduce an array of elements, emitted with the same key, to a single document. So, it accepts an array. You're emitting each key only once, so it's an array with a single element with it.

Now you'll be able to call your MapReduce normally:

db.orders.mapReduce(map, reduceFunc2, {out: {inline: 1}});

The way you overridden emit function is broken, so you shouldn't use it.

Update. Mongo may skip reduce operation if there is only one document associated with the given key, because there is no point in reducing a single document.

The idea of MapReduce is that you maps each document into an array of key-value pairs to be reduced on the next step. If there is more than one value associated with the given key, Mongo runs a reduce operation to reduce it to the single document. Mongo expects reduce function to return reduced document in the same format as the elements which was emitted. It's why Mongo may run reduce operation any number of times for each key (up to the number of emits). There is also no guarantee that reduce operation will be called at all if there is nothing to reduce (e.g. if there is only one element).

So, it's best to move map logic to the proper place.

Update 2. Anyway, why are you using MapReduce here? You can just query for the documents you need:

db.orders.find({}, {
  items: {
    $elemMatch: {
      qty: {$gt: 5},
      sku: 'qqq'
    }
  }
})

Update 3. If you really want to do it with MapReduce, try this:

db.runCommand({
  mapreduce: 'orders',
  query: {
    items: {
      $elemMatch: {
        qty: {$gt: 5},
        sku: 'ggg'
      }
    }
  },
  map: function map (){
    this.items = this.items.filter(function (entry) {
      return (entry.qty>5 && entry.sku=='ggg')
    });
    emit(this._id,this);
  },
  reduce: function reduce (key, values) {
    return values[0];
  },
  verbose: true,
  out: {
    merge: 'map_reduce'
  }
})