nativemapreduce.js

map = function() {
  return this.tagsL.forEach(function(tagL, i) {
    return emit(tagL, 1);
  });
};

reduce = function(k, vals) {
  print (k)
  print (vals)
  return vals.length;
};

db.works.mapreduce(map, reduce)

tagaggregate.coffee

Works.aggregate [
  $unwind: '$tagsL'
,
  $group: _id: '$tagsL', c: {$sum: 1}
], (err, tags) ->
  return done err  if err
  console.log tags # [ { _id: 'javascript', c:40 }, etc... ]

tagmapreduce.coffee

o = {}
o.map = ->
  if @tagsL
    @tagsL.forEach (tagL, i) ->
      emit tagL, 1
  else
    return
o.reduce = (k, vals) ->
  Array.sum vals
o.out = inline: 1
o.verbose = true
mongoose.models.Work.mapReduce o, (err, tagsArray, stats) ->
  return done err  if err
  console.log "Tag Count took %d ms", stats.processtime
  tags = {}
  async.forEach tagsArray, (tag, cb) ->
    tags[tag._id] = tag.value
    cb()
  , (err) ->
    tags #a key value list of tag: count

tagmapreducewrong.coffee

o = {}
o.map = ->
  if @tagsL
    @tagsL.forEach (tagL, i) ->
      emit tagL, 1
  else
    return
o.reduce = (k, vals) ->
  vals.length
o.out = inline: 1
o.verbose = true
mongoose.models.Work.mapReduce o, (err, tagsArray, stats) ->
  return done err  if err
  console.log "Tag Count took %d ms", stats.processtime
  tags = {}
  async.forEach tagsArray, (tag, cb) ->
    tags[tag._id] = tag.value
    cb()
  , (err) ->
    tags #a key value list of tag: count