Russian doll caching in Ruby on Rails - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to cache a post partial using Russian doll caching.

Ruby on Rails

<%= cache(@post) do %>
  <%= [1] 'post' %>
<% end %>

Drag options to blanks, or click blank then click option'

Alink_to

Brender

Cform_for

Dcontent_tag

Attempts:

3 left

💡 Hint

Common Mistakes

Using link_to instead of render inside the cache block.

Forgetting to render the partial inside the cache block.

The render method is used inside the cache block to render the post partial, enabling Russian doll caching.

2fill in blank

medium

Complete the code to cache a collection of comments with Russian doll caching.

Ruby on Rails

<%= cache(@comments) do %>
  <%= [1] @comments %>
<% end %>

Drag options to blanks, or click blank then click option'

Aform_for

Blink_to

Crender

Dcontent_tag

Attempts:

3 left

💡 Hint

Common Mistakes

Using link_to or other helpers instead of render.

Not rendering the collection inside the cache block.

Rendering the collection inside the cache block caches each comment efficiently using Russian doll caching.

3fill in blank

hard

Fix the error in the code to properly use Russian doll caching for a post and its comments.

Ruby on Rails

<%= cache(@post) do %>
  <h2><%= [1] %></h2>
  <%= render @post.comments %>
<% end %>

Drag options to blanks, or click blank then click option'

Apost.title

Bpost[:title]

Ctitle

[e-mail]

Attempts:

3 left

💡 Hint

Common Mistakes

Using undefined variable post instead of @post.

Using incorrect syntax like post[:title].

Using @post.title correctly accesses the post's title inside the cache block.

4fill in blank

hard

Fill both blanks to use a cache key with a timestamp for Russian doll caching.

Ruby on Rails

<%= cache([@post, [1]]) do %>
  <%= render 'post', post: @post %>
<% end %>

Drag options to blanks, or click blank then click option'

[e-mail]_at

[e-mail]_at

[e-mail]

DTime.now

Attempts:

3 left

💡 Hint

Common Mistakes

Using @post.created_at which doesn't change after creation.

Using Time.now which changes every time and breaks caching.

Using @post.updated_at as part of the cache key ensures the cache updates when the post changes.

5fill in blank

hard

Fill all three blanks to implement nested Russian doll caching for a post and its comments.

Ruby on Rails

<%= cache(@post) do %>
  <h1><%= [1] %></h1>
  <ul>
    <% @post.comments.each do |comment| %>
      <li><%= [2](comment) do %>
        <p><%= [3] %></p>
      <% end %></li>
    <% end %>
  </ul>
<% end %>

Drag options to blanks, or click blank then click option'

[e-mail]

Bcache

Ccomment.body

Drender

Attempts:

3 left

💡 Hint

Common Mistakes

Not caching each comment individually.

Using render instead of cache for comments.

Not displaying the comment body correctly.

The post title is shown, each comment is cached with cache(comment), and the comment body is displayed inside.

Practice

1. What is the main purpose of Russian doll caching in Rails?

easy

A. To cache nested parts of a page separately for faster updates

B. To cache the entire page as a single block

C. To disable caching for nested components

D. To cache only the database queries

Solution

1. Step 1: Understand caching scope

   Russian doll caching caches nested parts of a page separately, not the whole page at once.

2. Step 2: Identify the benefit

   This allows only changed parts to be updated, speeding up page rendering.

3. Final Answer:

   To cache nested parts of a page separately for faster updates -> Option A

4. Quick Check:

   Russian doll caching = nested caching [OK]

Hint: Think nested caching updates only changed parts [OK]

Common Mistakes:

• Confusing with full page caching
• Thinking it disables caching
• Assuming it caches only queries

2. Which syntax correctly uses Russian doll caching in a Rails view?

easy

A. <% cache @post do %> <% cache @comment do %> ... <% end %> <% end %>

B. <% cache do @post, @comment %> ... <% end %>

C. <% cache @post + @comment do %> ... <% end %>

D. <% cache @post && @comment do %> ... <% end %>

Solution

1. Step 1: Check nested cache blocks

   Russian doll caching uses nested cache blocks for each object separately.

2. Step 2: Validate syntax correctness

   <% cache @post do %> <% cache @comment do %> ... <% end %> <% end %> correctly nests cache @post and inside it cache @comment blocks.

3. Final Answer:

   <% cache @post do %> <% cache @comment do %> ... <% end %> <% end %> -> Option A

4. Quick Check:

   Nested cache blocks = correct syntax [OK]

Hint: Look for nested cache blocks with separate objects [OK]

Common Mistakes:

• Trying to cache multiple objects in one block incorrectly
• Using operators like + or && inside cache call
• Not nesting cache blocks properly

3. Given this Rails view code snippet, what will happen when @comment is updated but @post is unchanged?

<% cache @post do %>
  <p>Post content</p>
  <% cache @comment do %>
    <p>Comment content</p>
  <% end %>
<% end %>

medium

A. Both post and comment caches are expired and re-rendered

B. Only the comment cache is expired and re-rendered

C. Only the post cache is expired and re-rendered

D. Neither cache is expired; page stays the same

Solution

1. Step 1: Understand cache dependencies

   Russian doll caching nests caches so inner caches expire independently from outer caches.

2. Step 2: Apply update effect

   Updating @comment expires only the comment cache, not the post cache.

3. Final Answer:

   Only the comment cache is expired and re-rendered -> Option B

4. Quick Check:

   Inner cache update = inner cache expired [OK]

Hint: Only changed nested cache re-renders, outer stays [OK]

Common Mistakes:

• Assuming outer cache always expires with inner
• Thinking no cache expires on update
• Confusing which cache block is affected

4. Identify the error in this Rails view code using Russian doll caching:

<% cache @post do %>
  <p>Post content</p>
<% cache @comment %>
  <p>Comment content</p>
<% end %>
<% end %>

medium

A. Extra end keyword causing syntax error

B. Nested cache blocks cannot be used

C. Incorrect variable names for caching

D. Missing block for inner cache call

Solution

1. Step 1: Check inner cache syntax

   The inner cache @comment call lacks a block (no do ... end).

2. Step 2: Understand block requirement

   Cache calls must have a block to define cached content; missing block causes error.

3. Final Answer:

   Missing block for inner cache call -> Option D

4. Quick Check:

   Cache calls need blocks [OK]

Hint: Always use do ... end with cache blocks [OK]

Common Mistakes:

• Omitting block for cache call
• Misplacing end keywords
• Assuming cache works without block

5. You want to implement Russian doll caching for a blog page with posts and comments. Which approach best ensures only changed comments or posts update their cache without re-rendering the whole page?

hard

A. Use cache @post.comments once for all comments without nesting

B. Cache the entire blog page in one cache block with a combined key

C. Nest cache @post do blocks around each post and inside nest cache comment do blocks for each comment

D. Disable caching for comments and cache only posts

Solution

1. Step 1: Understand nested caching strategy

   Russian doll caching works best by nesting cache blocks for each object separately.

2. Step 2: Apply to posts and comments

   Wrapping each post in a cache block and nesting comment cache blocks inside allows independent expiration.

3. Step 3: Evaluate other options

   Other options either cache too broadly or disable caching, losing efficiency.

4. Final Answer:

   Nest cache @post do blocks around each post and inside nest cache comment do blocks for each comment -> Option C

5. Quick Check:

   Nested cache blocks for each object = best practice [OK]

Hint: Nest cache blocks per object for fine-grained updates [OK]

Common Mistakes:

• Caching entire page at once
• Caching all comments together without nesting
• Disabling caching for parts