has_secure_passwordでauthenticateメソッドが追加される仕組み ref: http://qiita.com/acro5piano/items/416c08a808c850e0477e

active_record.rb

require_relative './secure_password'

class ActiveRecord
  extend SecurePassword
end

file0.txt

class User < ActiveRecord::Base
  has_secure_password
end

user = User.new
user.authenticate('password')

file4.txt

require_relative './user.rb'

user = User.new
puts user.authenticate
#=> true

file5.txt

class User < ActiveRecord
  has_secure_password

  def test
    has_secure_password
  end
end

user = User.new
puts user.authenticate
puts user.test
# => undefined method `has_secure_password'
#    for #<User:0x0055b91a9f0280> NoMethodError

secure_password.rb

module SecurePassword
  def has_secure_password
    include SecurePassword::InstanceMethodsOnActivation
  end

  module InstanceMethodsOnActivation
    def authenticate
      true
    end
  end
end

user.rb

require_relative './active_record'

class User < ActiveRecord
  has_secure_password
end