Get a list of all unique tags in mongodb

Question

I am beginning with mongodb and have a collection with documents that look like the following

{
    \"type\": 1,
    \"tags\": [\"tag1\", \"tag2\", \"tag3\"]         


        

Answer 1

If you just want a (distinct) list of the tags then using distinct will be best. Map/Reduce will be slower and can't use an index for the javascript part.

http://docs.mongodb.org/manual/reference/method/db.collection.distinct/

db.coll.distinct("tags", {type:1}) Will return a set of tags for type=1.

Answer 2

You are right, a Map/Reduce might work for what you are trying to accomplish, but a Set might be faster and less code.

> m =     function() {
...         for (var tag in this.tags) {
...             emit(this.tags[tag], 1);
...         }
...     }

> r =     function(key, values) {
...         return 1;
...     }

> db.tags.mapReduce(m, r).find()
{ "_id" : "tag1", "value" : 1 }
{ "_id" : "tag2", "value" : 1 }
{ "_id" : "tag3", "value" : 1 }