Caching by RoR book. Can you help me figure it out?

Question

Software testing

Please help me understand this code

<% if notice %>
<p id="notice"><%= notice %></p>
<% end %>

<h1>Your Pragmatic Catalog</h1>

<% cache ['store', Product.latest] do %>
  <% @products.each do |product| %>
    <% cache ['entry', product] do %>
      <div class="entry">
        <%= image_tag(product.image_url) %>
        <h3><%= product.title %></h3>
        <%= sanitize(product.description) %>
        <div class="price_line">
          <span class="price"><%= number_to_currency(product.price) %></span>
<!-- START_HIGHLIGHT -->
          <%= button_to 'Add to Cart', line_items_path(product_id: product) %>
<!-- END_HIGHLIGHT -->
        </div>
      </div>
    <% end %>
  <% end %>
<% end %>

According to the book, I didn’t understand anything at all, what and at what stage is cached?
And this is the code:

def self.latest
    Product.order(:updated_at).last
  end

Answer 1

This caches snippets for each individual product and a snippet for the entire list of products.

def self.latest
  Product.order(:updated_at).last
end

- returns the last modified product (selection by update date).
Fragment for a single product, invalidated when product changes:

<% cache ['entry', product] do %>
  ...
<% end %>

Snippet for the whole list:

<% cache ['store', Product.latest] do %>
  ...
<% end %>

is invalidated when Product.latest changes, that is, in fact, when any product changes.
When you revisit the code, the last change to the product (Product.latest) is checked. If there were no changes after caching, the full list is taken from the cache. If there were, it is re-rendered, while the changes of each product are checked in a cycle and, if they are absent, a fragment for it is taken from the cache.

Answer 2

There is no point in caching erb, see for yourself the rendering time of the view - it will not be faster for you to access the cache store. Cache the data in the controller.