Ruby on Rails Help Needed

apoch632

I am having trouble writing a login in Ruby on Rails.
This is the error I am getting
TypeError (can't convert nil into String):
app/models/user.rb:14:in `+'
app/models/user.rb:14:in `hash_password'
app/models/user.rb:28:in `authenticate' app/controllers/user_controller.rb:24:in `login'

My code in the .rb files is as follows
user_controller.rb

def login
    if request.post?
      user = User.authenticate(params[:user][:login], params[:user][:password])
      if user
        session[:user] = user.id
        flash[:notice] = "Login Successful"
        redirect_to :controller=>'user',:action=>'home'
      else
        flash[:error] = "Login Unsuccessful"
        redirect_to :controller=>'home'
      end
    end
  end

user.rb

def password=(pass)
    @password=pass
    self.password_salt = User.random_string(10) if !self.password_salt?
    self.password_hash = User.hash_password(@password, self.password_salt)
  end

protected
  def self.hash_password(pass,password_salt)
    Digest::SHA1.hexdigest(pass + password_salt)
  end

def self.random_string(len)
    #generate a random password consisting of strings and digits
    chars = ("a".."z").to_a + ("A".."Z").to_a + ("0".."9").to_a
    newpass = ""
    1.upto(len) { |i| newpass << chars[rand(chars.size-1)]}
    return newpass
  end

def self.authenticate(login,pass)
    u=find(:first, :conditions=>["login = ?", login])
    return nil if u.nil?
    return u if User.hash_password(pass,u.password_salt) == u.password_hash
    nil
  end

I am working through an example in a book so I am just getting to grips with Ruby on Rails
Cheers for any help