How To: Write Your Own Serializer - Devise Wiki

To create your own serializer, just change this:

ruby

  config.warden do |manager|
    manager.serialize_into_session(:your_scope) do |user|
      user.id
    end

    manager.serialize_from_session(:your_scope) do |key|
      User.find(key)
    end
  end

in your /config/initializers/devise.rb file.

For example, if you have more than one class in one scope that needs to be serialized and deserialized without getting the wrong class, you can do something like this:

ruby

  config.warden do |manager|
    manager.serialize_into_session(:user) do |user|
      [user.class, user.id]
    end

    manager.serialize_from_session(:user) do |keys|
      klass, id = keys
      klass.constantize.find(id)
    end
  end

In this specific case, we were using our own warden strategies without any authenticatable strategy from devise.