# Rename Fields
# false for upsert
# true for multiple documents
db.<collection_name>.update({"<original_key.h1.h2>": {$exists: true}}, {$rename:{"<original_key.h1.h2>":"<new_name>"}}, false, true);

# Merge two collections
db.c1.find().forEach(function(item) {
    db.c2.insert(item);
    db.c1.remove(item);
});

# Get Unique keys in a collection
mr = db.runCommand({
  "mapreduce" : "my_collection",
  "map" : function() {
    for (var key in this) { emit(key, null); }
  },
  "reduce" : function(key, stuff) { return null; }, 
  "out": "my_collection" + "_keys"
})
db[mr.result].distinct("_id")
["foo", "bar", "baz", "_id", ...]