Step 1

Open MongoVUE and connect to the server that contains the collection “cities”

Step 2

Right-click on “cities” collection under “Database Explorer”, and select “MapReduce”. This will launch the MapReduce view.

Step 3

Write the JavaScript code for Map function in “Map” tab.

Step 4

Go to “Reduce” tab and enter your JavaScript Reduce code.

Step 5

Go to “Finalize” tab and enter your JavaScript Finalize code.

Step 6

Go to “In & Out” tab. Enter the Json code under “{Query}” to exclude cities from USA.

Step 7

We are almost done, but before we run this program, let’s just save it to disk first. Click on the small arrow next to “Go!” button and select “Save As”. You will be prompted for a filename. Enter a suitable name, and your MapReduce code will be saved to a corresponding “.vumr” file on your computer.

Step 8

We are ready to roll. Just click the “Go!” button, and that’ll start the MapReduce operation. At the end, you’ll see the results in the bottom pane. You can also check the time taken in the statusbar. Additionally, the shell command is also available under “Learn Shell” toolbox.

This completes our tutorial. You can check the Learn Shell toolbox, it displays the following command.

db.runCommand({ mapreduce: cities,
 map : function Map() {
	var key = this.CountryID;
	emit(key, {
		"data":
		[
			{
				"name" : this.City,
				"lat"  : this.Latitude,
				"lon"  : this.Longitude
			}
		]
	});
}
 reduce : function Reduce(key, values) {

	var reduced = {"data":[]};
	for (var i in values) {
		var inter = values[i];
		for (var j in inter.data) {
			reduced.data.push(inter.data[j]);
		}
	}

	return reduced;
}

 finalize : function Finalize(key, reduced) {

	if (reduced.data.length == 1) {
		return { "message" : "This Country contains only 1 City" };
	}

	var min_dist = 999999999999;
	var city1 = { "name": "" };
	var city2 = { "name": "" };

	var c1;
	var c2;
	var d;
	for (var i in reduced.data) {
		for (var j in reduced.data) {
			if (i>=j) continue;
			c1 = reduced.data[i];
			c2 = reduced.data[j];
			d = Math.sqrt((c1.lat-c2.lat)*(c1.lat-c2.lat)+(c1.lon-c2.lon)*(c1.lon-c2.lon));
			if (d < min_dist && d > 0) {
				min_dist = d;
				city1 = c1;
				city2 = c2;
			}
		}
	}

	return {"city1": city1.name, "city2": city2.name, "dist": min_dist};
}
 query : { "CountryID" : { "$ne" : 254 } }
 out : { inline : 1 }
 });


from:http://www.mongovue.com/2011/04/05/how-to-perform-mapreduce-operations-in-mongov