Low-level caching with Rails.cache - Mini Project: Build & Apply

Low-level caching with Rails.cache

📖 Scenario: You are building a simple Rails app that fetches and displays product prices. To make the app faster, you want to store the prices temporarily in a cache so the app doesn't have to fetch them every time.

🎯 Goal: Build a small Rails code snippet that uses Rails.cache to store and retrieve product prices, improving speed by avoiding repeated calculations.

📋 What You'll Learn

Create a hash called product_prices with exact product-price pairs

Create a variable called cache_key with the value "product_prices_cache"

Use Rails.cache.fetch with cache_key to store product_prices if not cached

Add an expiration time of 5 minutes to the cache fetch

💡 Why This Matters

🌍 Real World

Caching is used in web apps to speed up repeated data access by storing results temporarily.

💼 Career

Understanding Rails low-level caching helps backend developers optimize app performance and reduce database load.

1

Create the product prices hash

Create a hash called product_prices with these exact entries: "apple" => 100, "banana" => 50, "orange" => 75.

Ruby on Rails

# Create the product_prices hash with exact entries
# Your code here

Hint

Use Ruby hash syntax with string keys and integer values.

2

Set the cache key

Create a variable called cache_key and set it to the string "product_prices_cache".

Ruby on Rails

product_prices = { "apple" => 100, "banana" => 50, "orange" => 75 }
# Create cache_key variable with value "product_prices_cache"
# Your code here

Hint

Just assign the string to the variable.

3

Fetch or store product prices in cache

Use Rails.cache.fetch with cache_key to get the cached product prices. If the cache is empty, store product_prices in the cache.

Ruby on Rails

product_prices = { "apple" => 100, "banana" => 50, "orange" => 75 }
cache_key = "product_prices_cache"
# Use Rails.cache.fetch with cache_key to cache product_prices
# Your code here

Hint

Use a block with Rails.cache.fetch to provide the value if cache is empty.

4

Add expiration time to cache fetch

Modify the Rails.cache.fetch call to add an expiration time of 5 minutes using the expires_in option.

Ruby on Rails

product_prices = { "apple" => 100, "banana" => 50, "orange" => 75 }
cache_key = "product_prices_cache"
cached_prices = Rails.cache.fetch(cache_key) { product_prices }
# Add expires_in: 5.minutes option to Rails.cache.fetch
# Your code here

Hint

Pass expires_in: 5.minutes as a second argument to Rails.cache.fetch.

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