ruby 41 lines · 5 steps

Caching patterns with Rails.cache.fetch

A catalog service that wraps expensive queries in Rails.cache.fetch, showing how keys, expiry, and invalidation work together.

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Rails Intermediate caching cache-keys expiry invalidation memoization
1class ProductCatalog
2 def featured_products
3 Rails.cache.fetch("catalog/featured_products", expires_in: 12.hours) do
4 Product.where(featured: true)
5 .where("stock_count > 0")
6 .order(popularity: :desc)
7 .limit(20)
8 .to_a
9 end
10 end
11 
12 def revenue_for(store, on:)
13 Rails.cache.fetch(["catalog/revenue", store.id, on.iso8601], expires_in: 1.hour) do
14 store.orders
15 .completed
16 .where(completed_on: on.all_day)
17 .sum(:total_cents)
18 end
19 end
20 
21 def category_tree
22 Rails.cache.fetch("catalog/category_tree", race_condition_ttl: 10.seconds) do
23 Category.includes(:subcategories).roots.map do |root|
24 { id: root.id, name: root.name, children: root.subcategories.pluck(:id, :name) }
25 end
26 end
27 end
28 
29 def price_for(product)
30 Rails.cache.fetch([product, "computed_price"]) do
31 base = product.base_price_cents
32 discount = product.active_promotions.sum(:discount_cents)
33 [base - discount, 0].max
34 end
35 end
36 
37 def invalidate(store)
38 Rails.cache.delete("catalog/featured_products")
39 Rails.cache.delete_matched("catalog/revenue/#{store.id}/*")
40 end
41end
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Rails.cache.fetch runs its block only on a cache miss and stores the return value under the given key.
  2. 2Cache keys should encode every input that changes the result, and array keys with model objects auto-version on update.
  3. 3Expiry, race_condition_ttl, and explicit deletion are complementary tools for keeping cached data fresh.

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