NodeJS password authentication

I'm starting now with NodeJs and I'm following a password authentication tutorial. However, I can not call the method that compares the passwords. In my model, I have this structure:

var UsuarioSchema = new Schema({
login: {
    type: String, 
    required: true,
    unique: true
},
password: {
    type: String,
    required: true
}
});

UsuarioSchema.pre('save', function(next){
var user = this;
if(this.isModified('password') || this.isNew){
    bcrypt.genSalt(10, function(err, salt){
        if(err){
            return next(err);
        }
        bcrypt.hash(user.password, salt, function(err, hash){
            if(err){
                return next(err);
            }
            user.password = hash;
            next();
        });
    });
}
else{
    return next();
}
});

UsuarioSchema.method.verificaSenha = function (password, cb){
bcrypt.compare(password, this.password, function(err, isMatch){
    if(err){
        return cb(err);
    }
    cb(null, isMatch);
});
}

module.exports = mongoose.model('Usuario', UsuarioSchema);

And on the route, where I do the POST, this:

router.post('/api/admin', function(req, res){
   Usuario.findOne({
       login: req.query.login
   }, function(err, user){
       if(err){
           res.json({sucesso: false, mensagem: 'Autenticação falhou. Usuário nao encontrado'});
       }
       else{
           user.verificaSenha(req.query.password, function(err, misMatch){
               if(isMatch && !err){
                   var token = jwt.encode(user, config.secret);
                   res.json({sucesso: true, mensagem: 'JWT: ' + token});
               }
               else{
                   res.send({sucesso: false, mensagem: 'Autenticação falhou. Senha inválida'});
               }
           });
       }
   });
});

But when I do POST, it returns me an error saying that "user.states" is not a function. How could you solve this problem? Thanks