mongoosevalidation虚拟

我有以下模型使用mongoose：

var Usuarios = function (){ var crypt = require('./crypt_helper'), _model, _findByEmail, _findById, _findByRememberToken, _schema, validadores = {} validadores.notBlank = function(val){ return !(/^\s*$/.test(val)); } validadores.email = function(val){ return /^[\w+\-.]+@[az\d\-.]+\.[az]+$/i.test(val); } validadores.unique = function(field, message){ _schema.path(field).validate(function(val, fn){ var query = {} query[field] = val; _model.find(query).exec(function (err, vals) { fn(err || vals.length === 0); }); }, message); }; var _schema = new mongoose.Schema({ nombres: { type : String, required: true, validate: [validadores.notBlank, 'no debe estar en blanco'] }, apellidos: { type : String, required: true, validate: [validadores.notBlank, 'no debe estar en blanco'] }, email: { type : String, index: { unique: true, required : true }, //required: true, lowercase: true, trim: true, validate: [validadores.email, 'no es un correo electrónico válido'] }, password_digest: { type : String, required: true, validate: [validadores.notBlank, 'no debe estar en blanco'] }, remember_token: String, token: String, fecha_token: Date, permisos: [String], tipo_id: String, documento: { type : String, // required: true, validate: [validadores.notBlank, 'no debe estar en blanco'] } }); _schema.virtual('password').set(function(pw){ this.password_digest = crypt.cryptPassword(pw); }); _schema.virtual('avatar').get(function(){ var gravatar_id = crypto.createHash('md5').update(this.email).digest('hex'); return "https://secure.gravatar.com/avatar/" + gravatar_id + '?s=' + s }); _schema.pre('save', function (next) { this.remember_token = crypt.token(); next(); }); _model = mongoose.model('usuarios', _schema); validadores.unique('email', 'Correo electrónico ya registrado'); validadores.unique('documento', 'Número de documento ya registrado'); _findByEmail = function(email, success, fail) { _model.findOne({'email': email}).exec(function(err, doc) { if(err){ fail(err); } else { success(doc); } }); }; _findByRememberToken = function (rt, success, fail) { _model.findOne({'remember_token': rt}).exec(function(err, doc) { if(err){ fail(err); } else { if(doc) { var u = doc.toJSON(); delete u['password_digest']; delete u['remember_token']; delete u['fecha_token']; delete u['token']; success(u); } else { success(); } } }); }; // Encontrar por ID _findById = function (id, success, fail) { _model.findOne({'_id': id}).exec(function(err, doc) { if(err){ fail(err); } else { if (doc){ var u = doc.toJSON(); delete u['password_digest']; delete u['remember_token']; delete u['fecha_token']; delete u['token']; success(u); } else { success(); } } }); }; return { model: _model, schema: _schema, findById: _findById, findByEmail: _findByEmail, findByRememberToken: _findByRememberToken }; }(); 

“密码”字段是虚拟属性，而“password_digest”则保存在数据库中。 我想在模型中validation密码的强度。 至less我想validation一下，比如6个字符。 我怎样才能做到这一点？