Fragment caching in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is fragment caching in Rails?

Fragment caching stores parts of a view to speed up page rendering by reusing cached HTML instead of regenerating it every time.

beginner

How do you create a fragment cache block in a Rails view?

Use cache do ... end around the HTML you want to cache in the view template.

beginner

Why is fragment caching useful in a web app?

It reduces server work by reusing parts of pages that don’t change often, making pages load faster for users.

intermediate

How can you expire a fragment cache in Rails?

You can expire it by changing the cache key or using methods like expire_fragment in controllers or background jobs.

intermediate

What is a cache key in fragment caching?

A cache key is a unique identifier for the cached fragment, often based on model data or timestamps to keep cache fresh.

Which Rails helper is used to wrap content for fragment caching?

A cache do ... end

B render partial

C content_for

D link_to

The cache do ... end helper wraps the content you want to cache as a fragment.

What happens if the cache key changes for a fragment cache?

A The server crashes

B The cached fragment is ignored and regenerated

C The cache is deleted permanently

D Nothing, cache stays the same

Changing the cache key forces Rails to regenerate the cached fragment with new content.

Fragment caching is best used when:

A Parts of the page rarely change

B Every part of the page changes every request

C You want to cache the entire page

D You want to cache database queries

Fragment caching speeds up rendering by caching parts that don’t change often.

Which method can be used to manually expire a fragment cache in Rails?

A clear_cache

B delete_cache

C reset_cache

D expire_fragment

The expire_fragment method removes a cached fragment so it can be regenerated.

What is a common way to generate a cache key for a fragment?

A Using current time only

B Using random numbers

C Using model id and updated_at timestamp

D Using user IP address

Using model id and updated_at timestamp ensures cache updates when data changes.

Explain how fragment caching works in Rails and why it improves performance.

Think about saving time by reusing parts that don’t change.

Describe how to implement and expire a fragment cache in a Rails view and controller.

Focus on how to wrap content and how to clear or update the cache.

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