title: Rails 4 - Rolify - expiry date on role instance
body: <p>I am trying to figure out how to use rolify in my Rails 4 app.</p>
  
  <p>I have a users table and a roles table. </p>
  
  <p>The roles table has the following:</p>
  
  <pre><code>create_table "roles", force: :cascade do |t|
      t.string   "name"
      t.integer  "resource_id"
      t.string   "resource_type"
      t.datetime "created_at"
      t.datetime "updated_at"
    end
  
    add_index "roles", ["name", "resource_type", "resource_id"], name: "index_roles_on_name_and_resource_type_and_resource_id", using: :btree
    add_index "roles", ["name"], name: "index_roles_on_name", using: :btree
  </code></pre>
  
  <p>I also have a user_roles join table with ids for each of user and role ids.</p>
  
  <p>I want to create some roles which have limited durations (the user's time in that role is limited rather than the role itself). If I appoint someone as a student, I will know their expected graduation date, so I can appoint them in the role student until around the graduation date. I want to add a renew function to allow them to continue beyond that date if I check to confirm that it is appropriate.</p>
  
  <p>I'm not sure whether to add an expiry date date attribute and a boolean renew? attribute on my roles model, or if there is a better way of approaching this. Does anyone have any ideas that might help?</p>