Russian doll caching in Ruby on Rails - Mini Project: Build & Apply

Russian Doll Caching in Rails

📖 Scenario: You are building a blog application where each Post has many Comments. To improve performance, you want to use Russian doll caching to cache posts and their comments efficiently.

🎯 Goal: Build a Rails view that uses Russian doll caching to cache each post and its comments. This will speed up page loading by reusing cached fragments when posts or comments have not changed.

📋 What You'll Learn

Create a list of posts with comments in the view

Add a cache key variable for posts

Use nested cache blocks for posts and their comments

Complete the view with proper cache blocks to enable Russian doll caching

💡 Why This Matters

🌍 Real World

Russian doll caching is used in real Rails apps to speed up page rendering by caching nested data efficiently, such as blog posts with comments or product listings with reviews.

💼 Career

Understanding Russian doll caching is important for Rails developers to optimize app performance and reduce server load in production environments.

1

Set up posts and comments data in the view

In the Rails view file, create a variable called posts that contains all posts with their associated comments loaded using Post.includes(:comments).

Ruby on Rails

# Your code here
posts = Post.includes(:comments)

Hint

Use Post.includes(:comments) to load posts with comments efficiently.

2

Create a cache key variable for each post

Add a variable called post_cache_key inside the loop that iterates over posts. Set it to post.cache_key to use Rails cache versioning for posts.

Ruby on Rails

posts = Post.includes(:comments)

posts.each do |post|
  # Create post_cache_key variable here
  # Your code here
end

Hint

Use post.cache_key to get a cache key that changes when the post updates.

3

Add nested cache blocks for posts and comments

Inside the posts.each do |post| ... end loop, add a cache block using cache(post_cache_key). Inside that block, add another cache block for each comment using cache(comment) while iterating over post.comments.

Ruby on Rails

posts = Post.includes(:comments)

posts.each do |post|
  post_cache_key = post.cache_key
  # Add cache blocks for post and comments here
  # Your code here
end

Hint

Use nested cache blocks: one for the post and one inside for each comment.

4

Complete the view with proper cache blocks

Wrap the entire posts list in a content_tag :div, id: 'posts' block. Ensure the nested cache blocks are inside this container to complete the Russian doll caching structure.

Ruby on Rails

# Wrap posts list in a div with id 'posts'
# Your code here
posts = Post.includes(:comments)

posts.each do |post|
  post_cache_key = post.cache_key
  cache(post_cache_key) do
    concat content_tag(:h2, post.title)
    post.comments.each do |comment|
      cache(comment) do
        concat content_tag(:p, comment.body)
      end
    end
  end
end

Hint

Use content_tag :div, id: 'posts' do to wrap the posts list and include all cache blocks 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