Index/Save conflict

conflict.js

var mongoose = require('mongoose'),
    Schema = mongoose.Schema;

mongoose.connect('mongodb://localhost/db');

var User = new Schema({
  email: { 'type': String, 'required': true, 'index': { 'unique': true, 'background': true } },
});

User = mongoose.model('User', User);

var newUser = new User();
newUser.email = 'john@doe.com';
newUser.save(function (err) {
  console.log('saved');
});
  
newUser = new User();
newUser.email = 'john@doe.com';
newUser.save(function (err) {
  console.log('saved');
});

model.js

Model.init = function () {
  // build indexes
  var self = this
    , indexes = this.schema.indexes
    , count = indexes.length;

  indexes.forEach(function (index) {
    self.collection.ensureIndex(index[0], index[1], function(){
      --count || self.emit('index');
      console.log('index finished');
    });
  });
};

output

$ node conflict.js
saved
saved
index finished