gistfile1.rb

class Room < ActiveRecord::Base
  has_many :auctions
end

class Auction < ActiveRecord::Base
  belongs_to :room
  validates :room, :presence => true
end

FactoryGirl.define do
  factory :room do
    group

    factory :room_with_auctions do
      after_build do |r| 
        r.auctions = [Factory.build(:auction, :room => nil)]
      end 
    end 
  end 
end

FactoryGirl.define do
  factory :auction do
    room
  end
end



    Failure/Error: let(:room) { Factory(:room_with_auctions) }
     ActiveRecord::RecordInvalid:
       Validation failed: Auctions is invalid

ree-1.8.7-2011.03 :005 > r = Factory(:room, :auctions => [a])
ActiveRecord::RecordInvalid: Validation failed: Auctions is invalid