How key-based cache expiration works for has_many relationship?

[Matthijs Langenberg]

As an opportunity to get my latest project so damn fast, I looked into key-based cache expiration, also known as the Russion Doll approach to caching.

There is something I don't understand.

"You deal with dependency structures by tying the model objects together on updates."

So if you change a todo that belongs to a todolist that belongs to a project, you update the updated_at timestamp on every part of the chain, which will automatically then update the cache keys based on these objects

Okay, so the hierarchy is project -> todolist -> todo. Then a single todo item, probably keeps track of its creator, right?

In Rails it could be declared like this:

class Project < ActiveRecord::Base

end

class Todolist < ActiveRecord::Base

  belongs_to :project, touch: true

end

class Todo < ActiveRecord::Base

  belongs_to :todolist, touch: true

  belongs_to :creator

end  

# in app/views/project/show.html.erb

<% cache project do %>

  <p>All my todo lists:</p>

  <%= render project.todolists %>

<% end %>

# in app/views/todolists/_todolist.html.erb

<% cache todolist do %>

  <p><%= todolist.name %>:</p>

  <%= render todolist.todos %>

<% end %>

# in app/views/todo/_todo.html.erb

<% cache todo do %>

  <p><%= todo.name %>(by <%= todo.creator.name %>)</p>

<% end %>

# This will not trigger todo.touch!,

# which in turn does not trigger todo.todolist.touch!

# and does not trigger todo.todolist.project.touch!

todo.creator.update_attributes(name: 'John Doe')

What do I need to change in order to make this process trivial to implement caching schemes and trust that I am never going to serve stale data?