Low-level caching with Rails.cache - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to write data to the cache with a key.

Ruby on Rails

Rails.cache.[1]("user_1", "John")

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

Awrite

Bdelete

Cread

Dfetch

Attempts:

3 left

💡 Hint

Common Mistakes

Using read instead of write to store data.

Using fetch when you want to just write without reading.

The write method stores data in the cache under the given key.

2fill in blank

medium

Complete the code to read cached data by key.

Ruby on Rails

user_name = Rails.cache.[1]("user_1")

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

Afetch

Bwrite

Cread

Ddelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using write instead of read to get data.

Using fetch when only reading is needed.

The read method retrieves data from the cache by key.

3fill in blank

hard

Fix the error in the code to fetch cached data or compute and store it.

Ruby on Rails

user_name = Rails.cache.[1]("user_1") { "John" }

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

Awrite

Bfetch

Cread

Ddelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using read which does not accept a block.

Using write which does not read existing data.

The fetch method reads cached data if present; otherwise, it runs the block, stores the result, and returns it.

4fill in blank

hard

Fill both blanks to write data with expiration time in seconds.

Ruby on Rails

Rails.cache.write("session_123", "active", expires_in: [1].[2])

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

A5

Bminutes

Cseconds

Dhours

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string like '5 seconds' instead of separate number and unit.

Using wrong units like 'hours' when seconds are intended.

The expires_in option sets how long the cache entry lasts. Here, 5 seconds means the data expires quickly.

5fill in blank

hard

Fill all three blanks to fetch cached data with a block and set expiration.

Ruby on Rails

user = Rails.cache.fetch("user_42", expires_in: [1].[2]) { [3] }

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

A10

Bminutes

CUser.find(42)

Dseconds

Attempts:

3 left

💡 Hint

Common Mistakes

Putting the block code outside the block braces.

Using wrong expiration units or values.

This code fetches user data from cache or runs User.find(42) to get it, caching it for 10 minutes (600 seconds).

Practice

1. What is the main purpose of using Rails.cache in a Rails application?

easy

A. To permanently save data to the database

B. To store data temporarily for faster repeated access

C. To send emails asynchronously

D. To handle user authentication

Solution

1. Step 1: Understand the role of caching

   Caching stores data temporarily to avoid repeating expensive operations.

2. Step 2: Identify what Rails.cache does

   Rails.cache provides a way to store and retrieve data quickly in memory or other stores.

3. Final Answer:

   To store data temporarily for faster repeated access -> Option B

4. Quick Check:

   Caching = Temporary storage for speed [OK]

Hint: Caching speeds up repeated data access by storing it temporarily [OK]

Common Mistakes:

• Confusing caching with permanent database storage
• Thinking caching handles user login
• Assuming caching sends emails

2. Which of the following is the correct syntax to fetch a cached value or compute and store it if missing using Rails.cache?

easy

A. Rails.cache.store(:key, expensive_method)

B. Rails.cache.get(:key) { expensive_method }

C. Rails.cache.fetch(:key) { expensive_method }

D. Rails.cache.read(:key) { expensive_method }

Solution

1. Step 1: Recall the fetch method usage

   Rails.cache.fetch(:key) { block } tries to read the key, or runs the block and stores the result if missing.

2. Step 2: Check other options for correctness

   get, store, and read do not accept a block to compute missing values automatically.

3. Final Answer:

   Rails.cache.fetch(:key) { expensive_method } -> Option C

4. Quick Check:

   Use fetch with block to cache missing data [OK]

Hint: Use fetch with a block to cache or compute missing data [OK]

Common Mistakes:

• Using read or get which don't accept blocks
• Trying to use store with a block
• Confusing fetch with read

3. Given the code below, what will be printed by the second Rails.cache.fetch('counter') call?

count = 0
Rails.cache.fetch('counter') do
  count += 1
end
puts Rails.cache.fetch('counter')

medium

A. nil

B. 0

C. 2

D. 1

Solution

1. Step 1: Understand the first fetch call

   The block runs because 'counter' key is missing, so count increments from 0 to 1 and stores 1.

2. Step 2: Understand the second fetch call

   The key 'counter' exists now, so fetch returns cached value 1 without running the block.

3. Final Answer:

   1 -> Option D

4. Quick Check:

   Cached value returned without block run = 1 [OK]

Hint: Cached value returns without running block again [OK]

Common Mistakes:

• Assuming block runs every time
• Thinking count resets to 0
• Expecting nil if key exists

4. Identify the error in this caching code snippet:

Rails.cache.fetch('user_1') do
  User.find(1)
end
Rails.cache.write('user_1')

medium

A. Calling write without a value argument causes an error

B. fetch block should not call User.find

C. The cache key must be a symbol, not a string

D. write should be called before fetch

Solution

1. Step 1: Analyze the write method usage

   Rails.cache.write requires a key and a value; here only the key is given, missing the value argument.

2. Step 2: Check other options for validity

   Using string keys is valid; fetch block can call User.find; write order is not mandatory here.

3. Final Answer:

   Calling write without a value argument causes an error -> Option A

4. Quick Check:

   write needs key and value [OK]

Hint: write needs both key and value arguments [OK]

Common Mistakes:

• Omitting value in write method
• Thinking keys must be symbols
• Misunderstanding fetch block usage

5. You want to cache a list of products but ensure the cache expires after 10 minutes. Which is the correct way to do this with Rails.cache.fetch?

hard

A. Rails.cache.fetch('products', expires_in: 10.minutes) { Product.all.to_a }

B. Rails.cache.fetch('products', expires: 10) { Product.all }

C. Rails.cache.write('products', Product.all, expires_in: 600)

D. Rails.cache.fetch('products', timeout: 600) { Product.all.to_a }

Solution

1. Step 1: Identify correct option for expiration

   expires_in: 10.minutes is the correct option to set cache expiration time in fetch.

2. Step 2: Check other options for correctness

   expires and timeout are not valid options; write can set expiration but fetch with block is preferred for caching computed data.

3. Final Answer:

   Rails.cache.fetch('products', expires_in: 10.minutes) { Product.all.to_a } -> Option A

4. Quick Check:

   Use expires_in with fetch for expiration [OK]

Hint: Use expires_in option with fetch to set cache lifetime [OK]

Common Mistakes:

• Using wrong option names like expires or timeout
• Not converting ActiveRecord relation to array
• Using write instead of fetch with block