anchor.diff:  import 'package:mongo_dart/mongo_dart.dart';
  +import 'dart:io' show Platform;
  +
  +var host = Platform.environment['MONGO_DART_DRIVER_HOST'] ?? '127.0.0.1';
  +var port = Platform.environment['MONGO_DART_DRIVER_PORT'] ?? '27017';
   
   main() async {
  -  Db db = new Db("mongodb://127.0.0.1/mongo_dart-blog");
  -  DbCollection collection;
  -  DbCollection usersCollection;
  -  DbCollection articlesCollection;
  +  Db db = new Db("mongodb://$host:$port/mongo_dart-blog");
     Map<String, Map> authors = new Map<String, Map>();
     Map<String, Map> users = new Map<String, Map>();
     await db.open();
  -  print(">> Dropping mongo_dart-blog db");
     await db.drop();
  -  print(
  -      "===================================================================================");
  +  print("====================================================================");
     print(">> Adding Authors");
  -  collection = db.collection('authors');
  +  var collection = db.collection('authors');
     await collection.insertAll([
       {
         'name': 'William Shakespeare',
  
positive.code:       'comments': [{'user_id': users['jdoe']["_id"], 'body': "great article!"}]
      }
    ]);
    print(
        "===================================================================================");
    print(">> Articles ordered by title ascending");
    await articlesCollection.find(where.sortBy('title')).forEach((article) {
      print(
          "[${article['title']}]:[${article['body']}]:[author_id: ${article['author_id']}]");
    });
    await db.close();
  }
  
negative.code:     },
      {'name': 'Jorge Luis Borges', 'email': 'jorge@borges.com', 'age': 123}
    ]);
    await db.ensureIndex('authors',
        name: 'meta', keys: {'_id': 1, 'name': 1, 'age': 1});
    await collection.find().forEach((v) {
      print(v);
      authors[v["name"]] = v;
    });
    print("====================================================================");
    print(">> Authors ordered by age ascending");
    await collection.find(where.sortBy('age')).forEach(
        (auth) => print("[${auth['name']}]:[${auth['email']}]:[${auth['age']}]"));
    print("====================================================================");
    print(">> Adding Users");
    var usersCollection = db.collection("users");
    await usersCollection.insertAll([
      {'login': 'jdoe', 'name': 'John Doe', 'email': 'john@doe.com'},
      {'login': 'lsmith', 'name': 'Lucy Smith', 'email': 'lucy@smith.com'}
    ]);
    await db.ensureIndex('users', keys: {'login': -1});
    await usersCollection.find().forEach((user) {
      users[user["login"]] = user;
      print(user);
    });
    print("====================================================================");
    print(">> Users ordered by login descending");
    await usersCollection.find(where.sortBy('login', descending: true)).forEach(
        (user) =>
            print("[${user['login']}]:[${user['name']}]:[${user['email']}]"));
    print("====================================================================");
    print(">> Adding articles");
    var articlesCollection = db.collection("articles");
    await articlesCollection.insertAll([
      {
        'title': 'Caminando por Buenos Aires',
  