Cache Keys for ActiveRecord Associations

app.models.concerns.collection.rb

module Collection
  extend ActiveSupport::Concern

  included do
    def cache_key(updated_at_field = :updated_at)
      class_name = self.class.to_s.demodulize.downcase
      count = self.count
      max_updated_at = self.maximum(updated_at_field).try(:utc).try(:to_s, :number)
      "#{class_name}/#{count}/#{max_updated_at}"
    end
  end
end

app.models.concerns.model.rb

module Model
  extend ActiveSupport::Concern

  included do
    def cache_key_for(prefix, collection = [], options = {})
      options.reverse_merge!(updated_at_field: :updated_at)
      key = "#{prefix}/#{self.cache_key}"

      if !collection.is_a?(Array)
        collection = [collection]
      end

      associations, records = collection.partition do |item|
        item.is_a?(ActiveRecord::Associations::CollectionProxy)
      end

      if records.present?
        key << cache_keys_to_append(records)
      end

      if associations.present?
        key << associations_cache_key(associations, options[:updated_at_field])
      end

      key
    end

    private

    def associations_cache_key(associations, updated_at_field)
      associations.collect { |association|
        association.cache_key(updated_at_field).prepend('/')
      }.join
    end

    def cache_keys_to_append(records)
      records.collect { |record|
        record.cache_key.prepend('/')
      }.join
    end
  end
end

app.models.example.rb

class Example < ActiveRecord::Base
  has_many :other_things
  has_one :thing
  
  def cache_key_for_other_things
    cache_key_for('other_things_partial', self.other_things)
  end
  
  def cache_key_for_thing_and_other_things
    cache_key_for('crazy_partial', [self.thing, self.other_things])
  end
end

config.initializers.active_record.rb

ActiveRecord::Base.class_eval do
  include Model
end

ActiveRecord::Associations::CollectionProxy.class_eval do
  include Collection
end