Why Low-level caching with Rails.cache? - Purpose & Use Cases

The Big Idea

What if your app could remember answers so it never asks the same question twice?

The Scenario

Imagine your web app fetches the same data from the database every time a user visits a page, even if the data hasn't changed.

This means slow page loads and extra work for your server.

The Problem

Manually repeating database queries wastes time and resources.

It makes your app slower and can frustrate users.

Also, writing code to remember and reuse data is tricky and easy to get wrong.

The Solution

Rails.cache lets you store data temporarily so your app can reuse it quickly without asking the database again.

This makes your app faster and reduces server load automatically.

Before vs After

✗ Before

user = User.find(1)
posts = Post.where(user_id: user.id).to_a

✓ After

posts = Rails.cache.fetch("user_1_posts", expires_in: 5.minutes) do
  Post.where(user_id: 1).to_a
end

What It Enables

You can speed up your app by reusing expensive data easily and reliably.

Real Life Example

A blog site caches the list of recent posts so visitors see pages load instantly without waiting for the database each time.

Key Takeaways

Manual repeated data fetching slows apps and wastes resources.

Rails.cache stores data temporarily to reuse it fast.

This improves speed and reduces server work effortlessly.

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