create.rb

require 'active_record'

ActiveRecord::Base.establish_connection adapter: "sqlite3", database: ":memory:"

class Article < ActiveRecord::Base
  has_one :comment
end

class Comment < ActiveRecord::Base
  validates :amazing, presence: true

  def amazing; ''; end
end

ActiveRecord::Base.connection.instance_eval do
  create_table(:articles)
  create_table(:comments) do |t|
    t.references :article
  end
end

a = Article.create!
a.build_comment
a.save!

a = Article.find a.id
p a => a.comment