title: Express Mongoose DB.once ('open') cannot execute a callback function
body: <pre><code>exports.c_39 = function(req,res) {
      var mongoose = require('mongoose');
      mongoose.createConnection('mongodb://localhost/cj');
      var db = mongoose.connection;
      db.on('error', console.error.bind(console, 'connection error:'));
      console.log('a')
      db.once('open',function(){
          console.log('b')
      })
  }
  </code></pre>
  
  <p>Can perform the console.log (' a '), but cannot perform DB.once ('open') callback function</p>