title: mongoose unique: true not work
body: <p>why mongoose unique not work at all in this script</p>
  
  <pre><code>  var child_process = require('child_process');
    // Load required packages
    child_process.exec("mongo test --eval 'db.users.drop();'", function(err){
    var mongoose = require('mongoose');
  
    console.log(mongoose.version);
    mongoose.connect('mongodb://localhost:27017/test');
    // Define our user schema
  
    var json = {};
    json.phone = { type: String, required: true, unique: true};
    var UserSchema = new mongoose.Schema(json);
    var Model = mongoose.model('user', UserSchema);
  
    var jp = new Model({ phone: "123456"});
    mongoose.connection.on('open', function(){
      console.log(jp);
      jp.save(function(err){
        console.log(err);
        var jp2 = new Model({ phone: "123456"});
        console.log(jp2);
        jp2.save(function(err){
          console.log(err);
          process.exit();
        });
      })
    });
  });
  </code></pre>
  
  <p>I'm quite confused, the result is like </p>
  
  <pre><code>3.8.20
  { phone: '123456', _id: 54856cceb5b40f7a88fcc2af }
  null
  { phone: '123456', _id: 54856cceb5b40f7a88fcc2b0 }
  null
  </code></pre>
  
  <p>Thank you for your help.</p>