Fragment caching in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this fragment cache block output?

Consider this Rails view snippet using fragment caching:

<% cache('user_42_profile') do %>
  <p>User profile content</p>
<% end %>

What will be rendered on the page the first time and on subsequent requests?

Ruby on Rails

<% cache('user_42_profile') do %>
  <p>User profile content</p>
<% end %>

A First request renders the content and caches it; later requests render the cached content without re-executing the block.

B Every request renders the content and updates the cache with new content.

C The cache block is ignored and content is always rendered fresh.

D The cache block raises an error because the key is a string.

💡 Hint

Think about how fragment caching stores rendered HTML to speed up future requests.

Fragment caching stores the rendered HTML the first time the block runs. Later requests use the cached HTML directly, skipping the block.

📝 Syntax

intermediate

Identify the syntax error in this fragment cache usage

Which option contains a syntax error in using fragment caching in Rails views?

A <% cache(user) do %>Content<% end %>

B <% cache 'user' Content <% end %>

C <% cache 'user' do %>Content<% end %>

D <% cache user do %>Content<% end %>

💡 Hint

Look for missing 'do' or incorrect ERB tags.

Option B is missing the 'do' keyword and has incorrect ERB tag placement, causing a syntax error.

🔧 Debug

advanced

Why does this fragment cache not update after model change?

Given this code snippet:

<% cache(@post) do %>
  <p><%= @post.title %></p>
<% end %>

After updating @post.title in the database, the cached fragment still shows the old title. Why?

Ruby on Rails

<% cache(@post) do %>
  <p><%= @post.title %></p>
<% end %>

A Fragment caching does not work with instance variables.

B Rails caches the fragment forever by default, ignoring model changes.

C The cache block must be manually expired after updating the model.

D The cache key is based on @post's cache_key method, which did not change because the record was not saved properly.

💡 Hint

Check how Rails generates cache keys for Active Record objects.

Rails uses the model's cache_key, which includes timestamp. If the record was not saved or timestamp not updated, cache key stays same, so cache is reused.

🧠 Conceptual

advanced

What is the purpose of cache digests in fragment caching?

In Rails fragment caching, what role do cache digests play?

A They compress cached fragments to save space.

B They encrypt cached fragments for security.

C They track dependencies of partials to expire caches automatically when partials change.

D They store user session data inside cached fragments.

💡 Hint

Think about how Rails knows when to expire cached fragments if partial templates change.

Cache digests generate fingerprints of partial templates. When partials change, digests change, causing cache expiration automatically.

❓ state_output

expert

How many cached fragments exist after this code runs?

Given this Rails view code:

<% ['a', 'b', 'c'].each do |letter| %>
  <% cache("fragment_#{letter}") do %>
    <p>Letter: <%= letter %></p>
  <% end %>
<% end %>

After rendering once, how many separate cached fragments are stored?

Ruby on Rails

<% ['a', 'b', 'c'].each do |letter| %>
  <% cache("fragment_#{letter}") do %>
    <p>Letter: <%= letter %></p>
  <% end %>
<% end %>

A 3 cached fragments, one for each letter 'a', 'b', and 'c'.

B 1 cached fragment containing all letters combined.

C No cached fragments because cache keys are dynamic strings.

D 6 cached fragments due to internal Rails caching of partials.

💡 Hint

Each cache block with a unique key creates one fragment cache.

Each iteration caches a separate fragment with a unique key, so 3 fragments total.