ruby矿中的Rspec测试总是返回false

Question

我有一个类用户如下

class User < ActiveRecord::Base

  has_one :session
  has_and_belongs_to_many :posts
  has_and_belongs_to_many :replies

  attr_accessor :clear_text_password;

  validates_presence_of :username
  validates_uniqueness_of :username
  validates :clear_text_password, :presence => true, :length => {:minimum => 6}
  validates_confirmation_of :clear_text_password

  validates_presence_of :e_mail
  validates_uniqueness_of :e_mail

  def User.authenticate(name, password)
    if user = find_by_username(name)
      if user.password == encrypt_password(password)
        user

      end
    end
  end

end 

现在，我的rspec测试文件如下

require 'spec_helper'
require 'rspec'
require 'rspec-rails'
require 'user'

describe User do
  describe "my first test" do
    it "should redirect if user is authenticated" do
      user = User.authenticate('abcd','abcdef')
      c=false
      if user
        c=true
      end
      c.should == true

    end
  end
end

数据库表用户的用户名和密码为“abcd”和“abcdef”。但测试总是失败。方法调用始终返回false。请帮忙!!