need to use `memoize` in custom module in rails3

I have a module in lib folder. Where I have created a InstanceMethods module.

require 'memoist'
extend Memoist
module MyStudentMethods
  def self.included base
    base.send :include, InstanceMethods
  end

  module InstanceMethods
    def find_student(user_id)
      self.student.find{|student| student.user_id == user_id }
    end
    memoize :find_student
  end
end

I am using this module in my model.

require_dependency 'my_student_methods'

class Student < ActiveRecord::Base
  # previously I was using memoize like below commented out code
  # require 'memoist'
  # extend Memoist
  include MyStudentMethods
end

But when I start rails server it through an error:

/home/Projects/school/lib/my_student_methods.rb:12:in `<module:InstanceMethods>': undefined method `memoize' for MyStudentMethods::InstanceMethods:Module (NoMethodError)

Issue: How can I memoize the module method?.