Hack the counter_cache

gistfile1.rb

# activerecord/lib/active_record/associations/builder/belongs_to.rb automagically creates 
# the private methods for you if you include counter_cache in your belongs_to association. 
# We simply override the basic behavior of these with our own conditions.
# 
# For more information, check out:
# https://github.com/rails/rails/blob/733bfa63f5d8d3b963202b6d3e9f00b4db070b91/activerecord/lib/active_record/associations/builder/belongs_to.rb
# Lines 23 - 44

class Inventory < ActiveRecord::Base
  belongs_to :user, counter_cache:true
  
  def owned?
    state == 1
  end
  
  private

    def belongs_to_counter_cache_after_create_for_user
      User.increment_counter("inventories_count", user.id) if self.owned?
    end

    def belongs_to_counter_cache_before_destroy_for_user
      User.decrement_counter("inventories_count", user.id) if self.owned?
    end
  
end