Async unique validation with expressjs and mongoose

gistfile1.js

var mongoose = require('mongoose')                                                                                                                       
    ,Schema = mongoose.Schema                                                                                                                            
    ,ObjectId = Schema.ObjectId;                                                                                                                         
                                                                                                                                                         
var userSchema = new Schema({                                                                                                                            
    email:  String,                                                                                                                                      
    password: String,                                                                                                                                    
    created: { type: Date, default: Date.now }                                                                                                           
});                                                                                                                                                      
                                                                                                                                                         
User = mongoose.model('User', userSchema);                                                                                                               
module.exports = User                                                                                                                                    
                                                                                                                                                         
User.schema.path('email').validate(function (value, respond) {                                                                                           
    User.findOne({ email: value }, function (err, user) {                                                                                                
        if(user) respond(false);                                                                                                                         
    });                                                                                                                                                  
}, 'This email address is already registered');