private void createGroupByData() {
DBCollection c = mongoTemplate.getDb().getCollection("group_test_collection");
c.save(new BasicDBObject("x", 1));
c.save(new BasicDBObject("x", 1));
c.save(new BasicDBObject("x", 2));
c.save(new BasicDBObject("x", 3));
c.save(new BasicDBObject("x", 3));
c.save(new BasicDBObject("x", 3));