How to user factory girl to create associated lists with a has_many with a validation that requires it on create

In a Rails application, given three models User, Article and Reviewer with the following relationships and validations:

class User < ActiveRecord::Base
  has_many :articles
  has_many :reviewers
end

class Reviewer < ActiveRecord::Base
  belongs_to :user
  belongs_to :article
end

class Article < ActiveRecord::Base
  belongs_to :user
  has_many :reviewers

  validate :has_reviewers?

  def has_reviewers?
    errors.add(:base, "article must have at least one reviewer.") if self.reviewers.blank?
  end
end

And the following factories using the newer DSL:

FactoryGirl.define do

  factory :user do
    name { (8...20).map{ ('a'..'z').to_a[rand(26)] }.join }
    age  { Kernel.rand(100) }
  end

  factory :article do
    body "This is the article content"
    title "This is the title"
    user
    after_create do |article|
      article.reviewers = create_list(:user, 2)
    end
  end

  factory :reviewer do
    user
    article
    state { ["published","draft","rejected","archived"][Kernel.rand(4)] }
  end

end

The factory to create the article doesn't work because the validation fails before the reviewers are created:

> FactoryGirl.create(:article)
ActiveRecord::RecordInvalid: Validation failed: article must have at least one reviewer.

I have made more attempts than I would like to admit trying to overcome this hurdle, but I am stuck! One idea I had was to create the reviewers like this:

  factory :article do
    body "This is the article content"
    title "This is the title"
    user
    reviewers {|a| [FactoryGirl.create(:reviewer, article: a)] }
  end

but in this context, the "a" is not the instance. So that doesn't work either, like it used to.