Unnecessary & slow query executed when using scope as condition

reproduce.rb

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

gemfile(true) do
  source 'https://rubygems.org'
  gem 'rails', '5.2.0'  # use correct rails version
  gem 'cancancan' # use correct cancancan version
  gem 'sqlite3' # use another DB if necessary
end

require 'active_record'
require 'cancancan'
require 'cancan/model_adapters/active_record_adapter'
require 'cancan/model_adapters/active_record_4_adapter'
require 'cancan/model_adapters/active_record_5_adapter'
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)

# create your tables here
ActiveRecord::Schema.define do
  create_table :books, force: true  do |t|
    t.integer :user_id
  end

  create_table :users, force: true  do |t|
    t.string :name
  end
end

class Book < ActiveRecord::Base
  belongs_to :user
end

class User < ActiveRecord::Base
  has_many :books
end

user = User.create!
5.times { Book.create(user: user) }

class Ability
  include CanCan::Ability

  def initialize(user)
    can :read, Book, user.books do |book|
      book.user_id == user.id
    end
  end
end

class BooksController
  include CanCan::ControllerAdditions

  def params
    {controller: "books", action: "index"}
  end

  def current_user
    User.first
  end
end

class ActiveRecord::Associations::CollectionProxy
  def load_target
    raise "No need to load association"
  end
end

class BugTest < Minitest::Test
  def test_bug
    controller = BooksController.new
    resource = CanCan::ControllerResource.new(controller)
    resource.load_resource
    controller.instance_variable_get(:@books)
  end
end