Rails Caching

Use Rails caching to reduce response time and database load while keeping content fresh. This skill focuses on fragment caching, low-level caching, Solid Cache, cache keys, and conditional GETs.

Fragment caching

Wrap view fragments that can be reused:

<% @products.each do |product| %>
  <% cache product do %>
    <%= render product %>
  <% end %>
<% end %>

Notes:

• Cache keys include a template digest; edits to the fragment invalidate it.
• Model versions (cache_key_with_version) help expire cached content when records change.

Low-level caching with Rails.cache

Use Rails.cache.fetch for expensive computations:

class Product < ApplicationRecord
  def competing_price
    Rails.cache.fetch("#{cache_key_with_version}/competing_price", expires_in: 12.hours) do
      Competitor::API.find_price(id)
    end
  end
end

Guidance:

• Prefer fetch to avoid separate read/write logic.
• Choose stable keys; use arrays/hashes when appropriate.
• Set expires_in for data that should age out.

Solid Cache (Rails 8+ default)

Solid Cache is a database-backed ActiveSupport cache store optimized for SSDs. It uses FIFO eviction and is enabled by default in Rails 8.0+.

Skip it when creating a new app:

rails new app_name --skip-solid

Example database config (separate cache DB):

production:
  primary:
    <<: *default
    database: storage/production.sqlite3
  cache:
    <<: *default
    database: storage/production_cache.sqlite3
    migrations_paths: db/cache_migrate

Cache keys

Objects should implement cache_key (Active Record provides it). Keys can be arrays or hashes:

Rails.cache.read(site: "mysite", owners: [owner_1, owner_2])

Notes:

• Storage backends may modify keys; don’t depend on backend-specific formats.

Conditional GETs

Leverage ETags and Last-Modified to return 304 Not Modified:

class ProductsController < ApplicationController
  def show
    @product = Product.find(params[:id])
    if stale?(last_modified: @product.updated_at.utc, etag: @product.cache_key_with_version)
      # normal response rendering
    end
  end
end

When to use this skill

• Cache expensive views or queries
• Decide between fragment and low-level caching
• Configure Solid Cache or other cache stores
• Implement cache keys and conditional GETs