CouchDB View equivalent of SUM & GROUP BY

Question

I have multiple CouchDB documents representing a timestamp with a property userId and tag (a user can have n timestamps and assign each one a tag). I want to query CouchDB by a specific userId and get a sorted list of tags the user has used (sorted by occurrence).

How would the view look like?

If I want to get a list of all tags sorted by occurrence (no matter from which user) or if I assume that there are only documents with the same userId, I would make it like so:

Map:

function(doc) {
  if(doc.tag) emit(doc.tag, 1); 
}

Reduce:

function(keys, values) {
  return sum(values);
}

But how do I group the result so that I can filter the query by a specific userId?

Answer 1

Answering my own question.

Seems like CouchDB supports arrays as keys. With that in mind it's pretty simple:

Map:

function(doc) {
  if(doc.tag) emit([doc.userId, doc.tag], 1); 
}

Reduce:

function(keys, values) {
  return sum(values);
}

The result is then sorted by userId & tag.