MongoDB Map - Reduce result not exactly - mongodb

Question

I had data structure in MongoDB as below
{
"_id" : ObjectId("523aab00045624a385e5f549"),
"name" : "English Book 29",
"SKU" : 1000549081,
"price" : 249000,
"image" : null,
"category_id" : ObjectId("523a7802b50418baf38b4575"),
"category_name" : "English Book",
"details" : {
"Title" : "Title 549081",
"Binding" : 1,
"Author" : "Author 0",
"Publication data" : 0.5263832447608386,
"Publisher name" : "Publisher name 14",
"Number of page" : 90
}
}
Binding of book has 2 values:
0 that means soft binding, and 1 that means hard binding. I write Map Reduce to statistics for each values.
var map = function()
{
for(var key in this.details)
{
if(key == 'Binding')
{
emit({name: key}, {
'data':
[
{
name: this.details[key],
count: 1
}
]
});
}
}
};
var reduce = function (key, values) {
var reduced = {};
for(var i in values)
{
var inter = values[i];
for(var j in inter.data)
{
if(typeof(reduced[inter.data[j].name]) != "undefined")
{
reduced[inter.data[j].name] += inter.data[j].count;
}
else
{
reduced[inter.data[j].name] = 1;
}
}
}
return reduced;
};
When I run with small data (50 records) result return exactly. But when I run it with real data (192000 records) result return Not exactly. The result as below
{
"_id" : {
"name" : "Binding"
},
"value" : {
"0" : 50,
"1" : 50
}
}
I checked return data when Map/Reduce done, result as below
"counts" : {
"input" : 192000,
"emit" : 192000,
"reduce" : 1920,
"output" : 1
},
What wrong with it. Welcome any suggestion, explanation.
Thanks and best regards,

Answer 1

After researching about Map/Reduce yesterday, I realized that, "Emit" send 100 elements once, and "Reduce" perform on this data set. So my above code is wrong because it only "SUM" on small data set.
Below that is my new code for Map-Reduce
var map = function ()
{
for(var key in this.details)
{
if(key == 'Binding')
{
var value = {};
value[this.details[key]] = 1;
emit(key, value);
}
}
}
var reduce = function (key, values)
{
var reduced = {};
for(var idx = 0; idx < values.length; idx++)
{
var inner = values[idx];
for (var j in inner)
{
if (typeof (reduced[j]) == 'undefined')
{
reduced[j] = 0;
}
reduced[j] += inner[j];
}
}
return reduced;
}
I post here for anyone who meet similar situation. Thanks for reading.