MongoDB Shell MapReduce sample

map_reduce_sample.js

// map function
m = function () {
  this.images.forEach( function(id) {
    emit(id, 1);
  })
}
// reduce function
r = function (id, count) {
  return Array.sum(count);
}
// create a image count in collection map_reduce_sample
db.albums.mapReduce(m, r, {out: "map_reduce_sample"})
// tag all images which are in use
db.map_reduce_sample.find().forEach(function(d) { 
  db.images.update({_id: d._id}, {$set: {in_use: true}}) 
})
// remove unused images
db.images.remove({in_use: {$exists: false}})
// remove in_use tag
db.images.update({}, {$unset: {in_use: ""}})