// A simple grouping of Assets that mirror GridFS documents
db.system.js.save(
  { _id: 'assetSizeBuckets',
    value: function() {
      return db.assets.group({
        keyf: function(asset) {
          if(asset.file_size < 10000) {
            return {small: true}
          } else if(asset.file_size < 100000) {
            return {medium: true}
          } else {
            return {large: true}
          }
        },
        initial: {tsize: 0.0, count: 0},
        reduce: function(asset,memo){
          memo.count++;
          memo.tsize += asset.file_size;
        },
        finalize: function(out){ out.avg_size = out.tsize / out.count }
      });
    }
  }
);

// Where an Asset is a document like this:
{
	"_id"       : ObjectId("4be2f7ac80faa9d88e000005"),
	"file_name" : "some_file.html",
	"file_size" : 16849,
	"file_type" : "text/html",
	"file_id"   : ObjectId("4be2f7ac80faa9d88e000006")
}