Russian doll caching in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Understanding nested cache behavior in Rails views

Given a Rails view with nested cache blocks implementing Russian doll caching, what will be the output behavior when the inner cache key changes but the outer cache key remains the same?

Ruby on Rails

<% cache ['outer', @post.updated_at] do %>
  Outer content
  <% cache ['inner', @comment.updated_at] do %>
    Inner comment content
  <% end %>
<% end %>

A Only the inner cache block is re-rendered; the outer cache block is reused from cache.

B Both inner and outer cache blocks are re-rendered because the inner key changed.

C Only the outer cache block is re-rendered; the inner cache block is ignored.

D Neither cache block is re-rendered; both are reused from cache regardless of key changes.

💡 Hint

Think about how nested caches work independently in Russian doll caching.

In Russian doll caching, nested cache blocks are cached separately. If the inner cache key changes, only the inner block is re-rendered. The outer block remains cached if its key is unchanged.

📝 Syntax

intermediate

Correct syntax for Russian doll caching in Rails partials

Which option shows the correct syntax to implement Russian doll caching in a Rails partial that caches a post and its comments separately?

A

<% cache @post do %>
  <%= render @post %>
  <% cache @post.comments %>
    <%= render @post.comments %>
  <% end %>
<% end %>

B

<%= cache @post do %>
  <%= render @post %>
  <%= cache @post.comments do %>
    <%= render @post.comments %>
  <% end %>
<% end %>

C

<% cache @post do %>
  <%= render @post %>
  <% cache @post.comments do %>
    <%= render @post.comments %>
  <% end %>
<% end %>

D

<%= cache @post do %>
  <%= render @post %>
  <% cache @post.comments %>
    <%= render @post.comments %>
  <% end %>
<% end %>

💡 Hint

Remember that cache with a block uses <% %> tags, not <%= %>.

The cache helper with a block should use ERB tags <% %> because it does not output content directly. Using <%= %> would output the return value of the block, which is not intended.

🔧 Debug

advanced

Identifying why nested Russian doll caching is not working

A developer notices that when a comment is updated, the inner cache block updates correctly, but the outer cache block never expires even when the post is updated. What is the most likely cause?

A The outer cache key does not include the post's <code>updated_at</code> timestamp, so it never expires.

B The inner cache block is missing a cache key, so it always re-renders.

C The developer used <code>cache_if</code> instead of <code>cache</code> for the outer block.

D The Rails cache store is disabled, so no caching happens at all.

💡 Hint

Check what keys are used for cache expiration.

If the outer cache key does not change when the post updates, the outer cache block will never expire. Including post.updated_at in the cache key ensures it expires when the post changes.

❓ state_output

advanced

Cache key generation with nested Russian doll caching

Given the following code, what is the cache key generated for the inner cache block?

<% cache ['post', post.id, post.updated_at] do %>
  <% cache ['comment', comment.id, comment.updated_at] do %>
    Comment content
  <% end %>
<% end %>

A A string like "views/post/123-20240101" ignoring the inner cache key.

B A string like "views/post/123-20240101/comment/456-20240102" combining both keys.

C A string like "views/post/123-20240101/comment/123-20240101" mixing post and comment IDs.

D A string like "views/comment/456-20240102" based only on the inner cache key.

💡 Hint

Each cache block generates its own key independently.

In Russian doll caching, each cache block generates its own cache key independently. The inner cache key is based only on the inner block's key, not combined with the outer block's key.

🧠 Conceptual

expert

Why use Russian doll caching in Rails applications?

Which of the following best explains the main advantage of using Russian doll caching in Rails views?

A It caches the entire page as a single fragment, making cache management simpler but less flexible.

B It allows caching of nested view fragments independently, so only changed parts are re-rendered, improving performance and reducing unnecessary rendering.

C It automatically invalidates all caches when any nested model changes, ensuring data consistency at the cost of performance.

D It replaces the need for database query caching by storing all query results in the view cache.

💡 Hint

Think about how nested caching affects rendering efficiency.

Russian doll caching caches nested view fragments separately. This means when a small part changes, only that part is re-rendered, saving time and resources compared to re-rendering the whole view.