MongoDB: What's a good way to get a list of all unique tags?

Question

What's the best way to keep track of unique tags for a collection of documents millions of items large? The normal way of doing tagging seems to be indexing multikeys. I will frequently need to get all the unique keys, though. I don't have access to mongodb's new "distinct" command, either, since my driver, erlmongo, doesn't seem to implement it, yet.

Answer 1

Even if your driver doesn't implement distinct, you can implement it yourself. In JavaScript (sorry, I don't know Erlang, but it should translate pretty directly) can say:

result = db.$cmd.findOne({"distinct" : "collection_name", "key" : "tags"})

So, that is: you do a findOne on the "$cmd" collection of whatever database you're using. Pass it the collection name and the key you want to run distinct on.

If you ever need a command your driver doesn't provide a helper for, you can look at http://www.mongodb.org/display/DOCS/List+of+Database+Commands for a somewhat complete list of database commands.