a.rb

gem 'activerecord', '5.2.0'
require 'active_record'
p AR: ActiveRecord.version

module ActiveRecord
  module ConnectionAdapters
    class ConnectionHandler
      def self.create_owner_to_pool
        Concurrent::Map.new(initial_capacity: 2) do |h, k|
          # Discard the parent's connection pools immediately; we have no need
          # of them
          ConnectionHandler.discard_unowned_pools(h)

          h[k] = Concurrent::Map.new(initial_capacity: 2)
        end
      end

      def initialize
        # These caches are keyed by spec.name (ConnectionSpecification#name).
        @owner_to_pool = ConnectionHandler.create_owner_to_pool

        # Backup finalizer: if the forked child never needed a pool, the above
        # early discard has not occurred
        ObjectSpace.define_finalizer self, ConnectionHandler.unowned_pool_finalizer(@owner_to_pool)
      end
    end
  end
end

ActiveRecord::ConnectionAdapters::ConnectionHandler.new
10.times{ GC.start }
p ObjectSpace.each_object(ActiveRecord::ConnectionAdapters::ConnectionHandler).to_a

Gemfile

source 'https://rubygems.org'
gem 'activerecord', '5.2.0'

Gemfile.lock

GEM
  remote: https://rubygems.org/
  specs:
    activemodel (5.2.0)
      activesupport (= 5.2.0)
    activerecord (5.2.0)
      activemodel (= 5.2.0)
      activesupport (= 5.2.0)
      arel (>= 9.0)
    activesupport (5.2.0)
      concurrent-ruby (~> 1.0, >= 1.0.2)
      i18n (>= 0.7, < 2)
      minitest (~> 5.1)
      tzinfo (~> 1.1)
    arel (9.0.0)
    concurrent-ruby (1.1.4)
    i18n (1.5.3)
      concurrent-ruby (~> 1.0)
    minitest (5.11.3)
    thread_safe (0.3.6)
    tzinfo (1.2.5)
      thread_safe (~> 0.1)

PLATFORMS
  ruby

DEPENDENCIES
  activerecord (= 5.2.0)

BUNDLED WITH
   1.17.1