cancan-policies.rb

class PostPolicy < Struct.new(:user, :post)
  def update?
    post.user == user
  end
  def destroy?
    false
  end
  def show?
    true
  end
end

class Ability
  include CanCan::Ability

  def initialize(user)
    @user = user || User.new
    can do |action, object|
      policy(object).send_public(policy_method(action))
    end
  end

  def policy_method(action)
    "#{action}?"
  end

  def policy(object)
    find_policy(object).new(@user, object)
  end

  def find_policy(object)
    if object.respond_to?(:policy_class)
      object.policy_class
    elsif object.class.respond_to?(:policy_class)
      object.class.policy_class
    else
      model = if object.respond_to?(:model_name)
        object.model_name
      elsif object.class.respond_to?(:model_name)
        object.class.model_name
      elsif object.is_a?(Class)
        object
      else
        object.class
      end
      Object.const_get("#{model}Policy")
    end
  end
end