title: Confusion between mongoose.connection() and mongoose.createConnection()
body: <p>I've been studying mongoose for three days and I'm a bit confused about the use of these two methods (i know that "mongoose.connection()" will be deprecated in the future...)</p>
  
  <p>The problem is: when I'm trying to convert (from "mongoose.connection()" to "mongoose.createConnection()") the action.js file of this example <a href="https://gist.github.com/2785463" rel="noreferrer">https://gist.github.com/2785463</a> it seems to not work for me...</p>
  
  <p>there's my code...</p>
  
  <pre><code>var mongoose = require('mongoose'),
  db = mongoose.createConnection('localhost', 'test');
  
  db.on('error', function () {
    console.log('Error! Database connection failed.');
  });
  
  db.once('open', function (argument) {
    console.log('Database connection established!');
  
    mongoose.connection.db.collectionNames(function (error, names) {
      if (error) {
        console.log('Error: '+ error);
      } else {
        console.log(names);
      };
    });
  });
  </code></pre>
  
  <p>and there's my terminal output (typing "node test.js" on my ubuntu terminal..)</p>
  
  <pre><code>Database connection established!
  
  /home/_user_/Scrivania/node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/server.js:437
      throw err;
            ^
  TypeError: Cannot call method 'collectionNames' of undefined
    at NativeConnection.&lt;anonymous&gt; (/home/_user_/Scrivania/test2.js:11:25)
    at NativeConnection.g (events.js:192:14)
    at NativeConnection.EventEmitter.emit (events.js:93:17)
    at open (/home/_user_/Scrivania/node_modules/mongoose/lib/connection.js:408:10)
    at NativeConnection.Connection.onOpen (/home/_user_/Scrivania/node_modules/mongoose/lib/connection.js:415:5)
    at Connection._open (/home/_user_/Scrivania/node_modules/mongoose/lib/connection.js:386:10)
    at NativeConnection.doOpen (/home/_user_/Scrivania/node_modules/mongoose/lib/drivers/node-mongodb-native/connection.js:47:5)
    at Db.open (/home/_user_/Scrivania/node_modules/mongoose/node_modules/mongodb/lib/mongodb/db.js:287:14)
    at Server.connect.connectCallback (/home/_user_/Scrivania/node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/server.js:235:7)
    at g (events.js:192:14)
  </code></pre>