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

Fragment caching in Rails views

📖 Scenario: You are building a blog page in a Rails app. The page shows a list of posts. To make the page faster, you want to cache each post's HTML fragment so Rails doesn't have to render it every time.

🎯 Goal: Build a Rails view that uses fragment caching to store each post's HTML snippet. This will speed up page loading by reusing cached HTML for posts that haven't changed.

📋 What You'll Learn

Create a list of posts as an array of hashes in the controller

Set a cache key prefix variable in the view

Use cache helper with the cache key prefix and post id in the view partial

Wrap the post content HTML inside the cache block in the partial

💡 Why This Matters

🌍 Real World

Fragment caching is used in Rails apps to speed up rendering of views by storing parts of HTML that don't change often. This reduces server load and improves user experience.

💼 Career

Understanding fragment caching is important for Rails developers to optimize app performance and scalability in real projects.

1

Set up posts data in the controller

In the controller, create a variable called @posts and assign it an array of hashes with these exact entries: { id: 1, title: 'Hello Rails' }, { id: 2, title: 'Fragment Caching' }, { id: 3, title: 'Speed Up Views' }.

Ruby on Rails

# Your code here

Hint

Use an instance variable @posts and assign it an array of hashes with the exact keys and values.

2

Add a cache key prefix variable in the view

In the view file, create a variable called cache_key_prefix and set it to the string 'posts_list'.

Ruby on Rails

# Your code here

Hint

Just assign the string 'posts_list' to a variable named cache_key_prefix.

3

Use cache helper with cache key prefix and post id

In the view, write a for loop iterating over @posts with variable post. Inside the loop, use the cache helper with the key [cache_key_prefix, post[:id]] to wrap the post's title inside an <h2> tag.

Ruby on Rails

# Your code here

Hint

Use a for loop with post over @posts. Use cache [cache_key_prefix, post[:id]] do to wrap the post title.

4

Complete the fragment caching with proper ERB tags

Ensure the view code uses proper ERB tags: open the Ruby code with <% %> and output the post title with <%= %>. The cache block should wrap the <h2> with the post title inside.

Ruby on Rails

  
    
  

# Your code here

Hint

Make sure to use ERB tags correctly: <% %> for Ruby code and <%= %> to output the post title.

Practice

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

easy

A. To manage database connections efficiently

B. To store user login information securely

C. To save parts of a page so they load faster next time

D. To compress images for faster download

Solution

1. Step 1: Understand fragment caching concept

   Fragment caching stores parts of a webpage to avoid regenerating them every time.

2. Step 2: Identify the main benefit

   This speeds up page rendering by reusing cached parts instead of rebuilding them.

3. Final Answer:

   To save parts of a page so they load faster next time -> Option C

4. Quick Check:

   Fragment caching = save parts for speed [OK]

Hint: Fragment caching speeds up page parts loading [OK]

Common Mistakes:

• Confusing caching with user authentication
• Thinking it compresses images
• Assuming it manages database connections

2. Which of the following is the correct syntax to use fragment caching in a Rails view?

easy

A. <% cache 'unique_key' do %> ... <% end %>

B. <% cache do 'unique_key' %> ... <% end %>

C. <% cache 'unique_key' %> ... <% end %>

D. <% cache 'unique_key' { ... } %>

Solution

1. Step 1: Recall correct fragment caching syntax

   The correct syntax uses cache 'key' do ... end block in ERB.

2. Step 2: Check each option

   <% cache 'unique_key' do %> ... <% end %> matches the correct block form. Others have wrong block usage or missing do.

3. Final Answer:

   <% cache 'unique_key' do %> ... <% end %> -> Option A

4. Quick Check:

   Use cache 'key' do block [OK]

Hint: Remember: cache needs 'do' block syntax [OK]

Common Mistakes:

• Omitting the 'do' keyword
• Using braces {} instead of do...end in ERB
• Placing the key inside the block instead of as argument

3. Given this Rails view code:

<% cache 'post_1' do %>
  <h1>Post Title</h1>
  <p>Post content here.</p>
<% end %>

What happens if the post content changes but the cache is not expired?

medium

A. The page shows an error about stale cache

B. The page shows the old cached content, not the updated post

C. Rails automatically updates the cache with new content

D. The cache is ignored and content always updates

Solution

1. Step 1: Understand cache behavior on data change

   Fragment cache stores rendered content and does not update unless expired or cleared.

2. Step 2: Identify effect of not expiring cache

   If cache is not expired, Rails serves the old cached content even if data changed.

3. Final Answer:

   The page shows the old cached content, not the updated post -> Option B

4. Quick Check:

   Cache stale without expiry = old content shown [OK]

Hint: Cache must be expired to show updated content [OK]

Common Mistakes:

• Assuming cache auto-updates on data change
• Expecting an error when cache is stale
• Thinking cache is ignored automatically

4. You have this fragment cache in your view:

<% cache 'user_42_profile' do %>
  <p>User profile info</p>
<% end %>

But after updating user data, the cache still shows old info. What is the likely problem?

medium

A. The cache key 'user_42_profile' was not changed or expired after update

B. The cache block syntax is incorrect and causes failure

C. Rails does not support fragment caching for user profiles

D. The view template is missing the cache helper method

Solution

1. Step 1: Check cache key usage

   Cache keys must change or be expired to reflect updated data.

2. Step 2: Identify why old data shows

   If key stays same and cache not expired, old cached content remains visible.

3. Final Answer:

   The cache key 'user_42_profile' was not changed or expired after update -> Option A

4. Quick Check:

   Cache key unchanged = stale cache shown [OK]

Hint: Expire or change cache key after data updates [OK]

Common Mistakes:

• Assuming syntax error causes stale cache
• Believing Rails disables caching for certain views
• Forgetting to use cache helper in views

5. You want to cache a list of comments for a post, but each comment can be edited independently. Which approach best ensures cache updates correctly?

hard

A. Cache only the post content and ignore comments

B. Cache the entire comments list with a single static key

C. Do not use fragment caching for comments to avoid stale data

D. Cache each comment fragment separately using a unique key per comment

Solution

1. Step 1: Understand caching granularity

   When parts update independently, caching each part separately allows selective expiration.

2. Step 2: Apply to comments list

   Caching each comment with a unique key lets you expire only changed comments, keeping others cached.

3. Step 3: Evaluate other options

   Single static key caches whole list, causing stale data if one comment changes. Avoiding caching loses speed benefits.

4. Final Answer:

   Cache each comment fragment separately using a unique key per comment -> Option D

5. Quick Check:

   Separate keys for independent parts = correct caching [OK]

Hint: Cache parts with unique keys for independent updates [OK]

Common Mistakes:

• Caching entire list causing stale comments
• Skipping caching due to fear of stale data
• Ignoring comment updates in cache strategy