Rails 5.1.5 AR uses 3 queries to destroy 2 records

rails_ar_bug.rb

# frozen_string_literal: true

begin
  require 'bundler/inline'
rescue LoadError => e
  warn 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  # gem 'rails', github: 'rails/rails'
  gem 'rails', '5.1.5rc1'
  gem 'sqlite3'
end

require 'active_record'
require 'minitest/autorun'
require 'logger'

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :parent_nodes, force: true do |t|
  end

  create_table :child_nodes, force: true do |t|
    t.integer :ancestor_id
    t.string :ancestor_type
  end
end

class ParentNode < ActiveRecord::Base
  has_one :child_node, as: :ancestor, dependent: :destroy

  accepts_nested_attributes_for :child_node
end

class ChildNode < ActiveRecord::Base
  belongs_to :ancestor, polymorphic: true, touch: true, optional: true
end

class BugTest < Minitest::Test
  def test_association_stuff
    parent_node = ParentNode.create!
    parent_node.build_child_node.save!

    c = count_queries do
      ParentNode.first.destroy
    end

    assert_equal 6, c
  end

  private

  def count_queries(&block)
    count = 0

    counter_f = lambda  { |_name, _started, _finished, _unique_id, payload|
      count += 1 unless payload[:name].in? %w[CACHE SCHEMA]
    }

    ActiveSupport::Notifications.subscribed(counter_f, 'sql.active_record', &block)

    count
  end
end