Associated objects creation failing in a callback

failing_callbacks_example.rb

require "rubygems"
require 'active_record'
require "sqlite3"

ActiveRecord::Base.establish_connection(
  :adapter => "sqlite3",
  :database  => "test.sqlite3"
)

ActiveRecord::Migration.class_eval do
  drop_table(:posts)
  create_table(:posts) do |t|
    t.column :title, :string
  end
  drop_table(:authors)
  create_table(:authors) do |t|
    t.column :name, :string
    t.column :post_id, :integer
  end
end

class Post < ActiveRecord::Base
  has_one :author

  before_save :add_author

  def add_author
    a = build_author(name: 'default name')
    a.valid? # callbacks need to return false to cancel later calbacks and rollback the transaction
  end

end

class Author < ActiveRecord::Base
  belongs_to :post

  # always invalid
  validate do |a|
    a.errors.add(:base,'bla')
  end
end

# Post.delete_all
# Author.delete_all

post=Post.create(title: 'primo post')

p Post.all
p Author.all

also consider:
http://www.samuelmullen.com/2013/05/the-problem-with-rails-callbacks/