explicit_counter_cache.rb

# frozen_string_literal: true

# Complex counter counter caches
# 
# Usage:
# 
# In parent model
#   class Post < ApplicationRecord
#     ExplicitCounterCache.define(self, votes_count, -> { votes.active })    
#     ExplicitCounterCache.define(self, comments_count, -> { votes.visible })  
#   end
# 
# In children models
#   class Vote < ApplicationRecord
#     ExplicitCounterCache::Refresh.define(self, :post, votes_count)    
#   end
# 
#   class Comment < ApplicationRecord
#     ExplicitCounterCache::Refresh.define(self, :comment, votes_count)    
#   end
#
module ExplicitCounterCache
  def self.define(model, counter_name, fetch_relation)
    model.define_method "refresh_#{counter_name}" do
      relation_or_count = instance_exec(&fetch_relation)
      new_count = relation_or_count.is_a?(Numeric) ? relation_or_count : relation_or_count.count

      return if self[counter_name] == new_count
      return if destroyed?

      # We intentionally avoid callbacks/validations here
      #   since these might be too expensive in some cases, e.g. checking
      #   another field for uniqueness even though we're just updating a counter.
      update_columns counter_name => new_count, updated_at: Time.current
    end
  end

  module Refresh
    def self.define(model, parent_model, counter_name, same_transaction: false)
      method_name = "refresh_#{parent_model}_#{counter_name}".to_sym

      model.define_method method_name do
        public_send(parent_model)&.public_send("refresh_#{counter_name}")
      end

      model.instance_eval do
        if same_transaction
          after_create method_name
          after_destroy method_name
        else
          after_commit method_name
        end
      end
    end
  end
end