Mongoose findOne函数未被执行

  ... (... = some unrelated code)

  var mongoose = require('mongoose');
  var db = mongoose.connect('mongodb://localhost/test');

  ...

  dataSchema = new Schema({
     'url': { type: String, index: true },
     'user_id': { type: Schema.ObjectId, index:true }
   });
  var Website = mongoose.model('websites', dataSchema);

  ...

  Website.findOne({url: "someurl.com"},function (err, docs) {
    console.log(docs._id);
  });

  ... 
  ... (... = some unrelated code)

  var mongoo