Try to run the conde in https://guides.rubyonrails.org/association_basics.html#options-for-has-many-source-type

error.log

Traceback (most recent call last):
	8: from ./sample.rb:49:in `<main>'
	7: from ./sample.rb:50:in `<class:Book>'
	6: from /Users/jnito/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/activerecord-6.0.0/lib/active_record/associations.rb:1511:in `has_one'
	5: from /Users/jnito/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/activerecord-6.0.0/lib/active_record/associations/builder/association.rb:30:in `build'
	4: from /Users/jnito/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/activerecord-6.0.0/lib/active_record/associations/builder/association.rb:40:in `create_reflection'
	3: from /Users/jnito/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/activerecord-6.0.0/lib/active_record/associations/builder/association.rb:67:in `validate_options'
	2: from /Users/jnito/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/activesupport-6.0.0/lib/active_support/core_ext/hash/keys.rb:50:in `assert_valid_keys'
	1: from /Users/jnito/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/activesupport-6.0.0/lib/active_support/core_ext/hash/keys.rb:50:in `each_key'
/Users/jnito/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/activesupport-6.0.0/lib/active_support/core_ext/hash/keys.rb:52:in `block in assert_valid_keys': Unknown key: :polymorphic. Valid keys are: :class_name, :anonymous_class, :foreign_key, :validate, :autosave, :foreign_type, :dependent, :primary_key, :inverse_of, :required, :as, :touch (ArgumentError)

has_one_polymprphic_association.rb

# frozen_string_literal: true

require "bundler/inline"

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

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

  # Activate the gem you are reporting the issue against.
  gem "activerecord", "6.0.0"
  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 :hardbacks, force: true do |t|
  end

  create_table :paperbacks, force: true do |t|
  end

  create_table :books, force: true do |t|
    t.integer :author_id
    t.string :format_type
    t.integer :format_id
  end

  create_table :authors, force: true do |t|
  end
end

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
end

class Author < ApplicationRecord
  has_many :books
  has_many :paperbacks, through: :books, source: :format, source_type: "Paperback"
end

class Book < ApplicationRecord
  has_one :format, polymorphic: true
  # Should change to belongs_to?
  # belongs_to :format, polymorphic: true
end

class Hardback < ApplicationRecord; end
class Paperback < ApplicationRecord; end

class AuthorTest < Minitest::Test
  def test_association
    author = Author.create!
    paperback = Paperback.create!
    book = Book.new
    book.format = paperback
    book.author_id = author.id
    book.save!

    author.reload
    assert_equal paperback, author.paperbacks.first
  end
end