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 b

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.