Fragment caching in Ruby on Rails - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to cache a fragment named 'menu' in a Rails view.

Ruby on Rails

<% cache [1] do %>
  <nav>Menu content here</nav>
<% end %>

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

A'menu'

Bmenu

C:menu

D"menu"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a symbol instead of a string for the cache key.

Omitting quotes around the cache key.

In Rails views, fragment caching uses a string key inside the cache helper, so 'menu' as a string is correct.

2fill in blank

medium

Complete the code to cache a fragment with a dynamic key using a variable 'product'.

Ruby on Rails

<% cache [1] do %>
  <div><%= product.name %></div>
<% end %>

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

Aproduct.id

B'product'

Cproduct

D[product, 'details']

Attempts:

3 left

💡 Hint

Common Mistakes

Using just 'product' which caches the whole object incorrectly.

Using product.id alone without context.

Using a plain string 'product' which is static.

Using an array like [product, 'details'] creates a cache key that changes when the product changes, which is best practice.

3fill in blank

hard

Fix the error in the fragment caching code by completing the blank.

Ruby on Rails

<% cache [1] do %>
  <p>Cached content here</p>
<% end %>

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

Aproduct.cache_key_with_version

Bproduct.cache_key

Cproduct.id

Dproduct.to_s

Attempts:

3 left

💡 Hint

Common Mistakes

Using product.id which does not change when product updates.

Using product.to_s which is not a cache key.

Using cache_key without version may cause stale cache.

Using product.cache_key_with_version ensures the cache key changes when the product or its version changes, avoiding stale cache.

4fill in blank

hard

Fill both blanks to cache a fragment with a custom expiration time of 10 minutes.

Ruby on Rails

<% cache({ key: [1], expires_in: [2] }) do %>
  <section>Expiring cache content</section>
<% end %>

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

A'sidebar'

B10.minutes

C5.minutes

D'header'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a number without .minutes for expiration.

Using a symbol instead of a string for the key.

Setting expiration time too short or too long.

The key 'sidebar' identifies the fragment, and expires_in: 10.minutes sets the cache to expire after 10 minutes.

5fill in blank

hard

Fill all three blanks to cache a fragment with a key based on 'user', include a version, and set expiration to 30 minutes.

Ruby on Rails

<% cache(key: [1], version: [2], expires_in: [3]) do %>
  <article>User profile content</article>
<% end %>

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

A[user]

Buser.cache_version

C30.minutes

Duser.id

Attempts:

3 left

💡 Hint

Common Mistakes

Using user.id alone without versioning.

Using a string instead of an array for the key.

Omitting expiration or version leading to stale cache.

The key is an array with user to generate a unique cache key, version uses user.cache_version for freshness, and expires_in sets 30 minutes expiration.

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