A simple guide to finding distinct array values in a MongoDB collection

So you want to find unique values within an array within a document in a collection? A reasonable request.

In ANSI SQL you'll be using DISTINCT, JOINS and GROUP BYs, stuff you're used to, but in the NoSQL realm your best bet is mapreduce.

It might seem a little bit like hard work, and probably a little intimidating at first, but it is certainly worth it; mapreduce is an extremely powerful tool.

Set up the collection, the map and reduce functions, and execute the mapreduce command:

	use itemCollection
	db.itemCollection.insert({
	"id" : "1",
	"somefield" : "blah",
	"someotherfield" : "meh",
	"notanotherfield" : "feh",
	"items" : ["item0", "item1", "item2", "item2"]
	})
	db.itemCollection.insert({
	"id" : "2",
	"somefield" : "bleh",
	"someotherfield" : "geh",
	"notanotherfield" : "beh",
	"items" : ["item0", "item1", "item2", "item1", "item0"]
	})
	db.itemCollection.insert({
	"id" : "3",
	"somefield" : "bluh",
	"someotherfield" : "heh",
	"notanotherfield" : "jeh",
	"items" : ["item0", "item1", "item2", "item3", "item4"]
	})
	map = function() {
	if (!this.items) {
	return;
	}
	for (index in this.items) {
	emit(this.items[index], 1);
	}
	}
	reduce = function(key, values) {
	var count = 0;
	for (index in values) {
	count += values[index];
	}
	return count;
	}
	db.runCommand( { mapreduce : "itemCollection", map : map , reduce : reduce , query : { "id" : "1"} , out : "items" } );
	db.items.find().forEach(printjson);
	{ "_id" : "item0", "value" : 1 }
	{ "_id" : "item1", "value" : 1 }
	{ "_id" : "item2", "value" : 2 }

view raw distinctarray1 hosted with ❤ by GitHub

But now you want to find unique values within an array within each document in an entire collection.

	db.runCommand( { mapreduce : "itemCollection", map : map , reduce : reduce, out : "items" } );
	db.items.find().forEach(printjson)
	{ "_id" : "item0", "value" : 4 }
	{ "_id" : "item1", "value" : 4 }
	{ "_id" : "item2", "value" : 4 }
	{ "_id" : "item3", "value" : 1 }
	{ "_id" : "item4", "value" : 1 }

view raw distinctarray2 hosted with ❤ by GitHub

The map function in this example iterates over each of the items and emits the key/value of each array element.

The reduce function aggregates the key/value from each of the emits from the map function. In this example we're looking at unique keys and maintaining a count of the unique keys.

If you're looking to find the distinct array elements for a single document, simply specify the document index. For the entire collection, just leave the query out. *simples*