Mongo distinct keys map reduce

distinct_keys.js

var collection = "your_collection"
var distinct_keys_map_reduce = db.runCommand({
  "mapreduce" : collection,
  "map" : function() {
    for (var key in this) { emit(key, null); }
  },
  "reduce" : function(key, val) { return null; }, 
  "out": collection + "_keys"
})
db[distinct_keys_map_reduce.keys].distinct("_id")