Ruby on Rails

I "discovered" some interesting functionality today. Let's say you have the following relationship:

class User
  has_many :posts
  accepts_nested_attributes_for :posts
end

class Post
  belongs_to :user, :touch => true
end

Editing a post as a child of a user touches the user record as expected:

user = User.first
user.posts.first.update(:title => "New Post Title")

But submitting that same change via nested attributes does NOT touch the user:

user = User.first
user.update(:posts_attributes => { '0' => { :id => user.posts.first.id, :title => 'New Post Title' } })

This seems really strange to me given the focus of Russian Doll Caching. Changing a child, in whatever manner you decide to do it, should touch the parent so cache keys update and caching Just Works™. Right?

Unless I'm missing something? I found one blog post on the entire internet that explicitly points out this functionality: http://www.software-thoughts.com/2014/03/rails-updating-association-through.html The single comment on that post says "just use :touch => true" which is what I assumed worked as well!

Rob