Poor man's caching for Jbuilder

_user.json.jbuilder

# app/views/users/_user.json.jbuilder
json.(user, :id, :name, :age, :gender)

application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery

  def cached_jbuilder(obj, opts = {})
    cache_key = (opts[:cache_key_prefix] || []) + Array.wrap(obj)
    cached = read_fragment(cache_key)

    if cached
      out = view_context.raw cached
    elsif obj.is_a?(Array)
      out = ""
      obj.each_index do |i|
        is_last = (obj.length - 1 == i)
        out += cached_jbuilder(obj[i], opts) + (!is_last ? "," : "")
      end
      out = "[#{out}]"
      write_fragment cache_key, out
      out
    else
      out = view_context.raw render_to_string partial: obj, locals: { obj.class.to_s.tableize.to_sym => obj }
      write_fragment cache_key, out
      out
    end
    view_context.raw out
  end

end

index.json.erb

<% # app/views/users/index.json.erb -%>
{"users":<%= controller.cached_jbuilder @users -%>}

The idea behind this is to start building the JSON output through an ERB template. We can then easily use the caching helpers to partially or entirely cache the resulting JSON.

In this example, if @users is populated with 3 records when we render index.json.erb, a total of 4 cache keys will be created: 3 JSON string representations of each user, and 1 JSON string containing all 3 users in an array. If one user changes, the array cache is busted, as well as the cache for the record that has changed.