new.rb

FactoryGirl.define do
  sequence :email do |n|
    "user#{n}@example.com"
  end

  factory :user, :aliases => [:author] do
    email
    password "test"
  end

  factory :post do
    author
    title "A post"
  end
end

old.rb

Factory.sequence(:email) { |n| "user#{n}@example.com" }

Factory.define :user do |factory|
  factory.email { Factory.next(:email) }
  factory.password "test"
end

Factory.define :post do |factory|
  factory.association :author, :factory => :user
  factory.title "A post"
end