Redis Counter Cache - easy and non-locking

counter.rb

# app/models/concerns/counter.rb
require 'active_support/concern'

module Counter
  module Updater
    extend ActiveSupport::Concern

    included do
      after_create :update_counters
    end

    def update_counters
      self.class.redis_counters.each do |relation_name|
        counter_name = "#{self.class.name.pluralize.underscore}_counter"
        self.send(relation_name).send(counter_name).increment
      end
    end

    module ClassMethods
      def redis_counters
        @counters ||= []
      end

      def update_counter_for(relation_name)
        redis_counters << relation_name
      end
    end
  end

  module Definition
    extend ActiveSupport::Concern

    included do
      include Redis::Objects
    end

    module ClassMethods
      def counter_for(relation_name)
        counter "#{relation_name}_counter"

        define_method "#{relation_name}_count" do
          self.send("#{relation_name}_counter").value
        end
      end
    end
  end
end

event.rb

# app/models/event.rb
class Event < ActiveRecord::Base
  include Counter::Definition
  
  has_many :inscriptions
  counter_for :inscriptions
end

inscriptions.rb

class Inscriptions < ActiveRecord::Base
  include Counter::Updater
  
  belongs_to :event
  update_counter_for :event
end