factories.rb

Factory.define :post do |post_factory|
  post_factory.user { Factory.build(:user) }
end

post.rb

class Post < ActiveRecord::Base
  belongs_to :user
  validates_presence_of :user_id
end

post_spec.rb

describe Post, "factory" do
  subject { Factory.build(:post) }
  it { should_not be_valid }
end

describe Post, "valid factory with stubbed ID" do
  subject { Factory.build(:post, :user_id => 1) }
  it { should be_valid }
end

describe Post, "valid factory with created association" do
  subject { Factory.build(:post, :user => Factory.create(:user)) }
  it { should be_valid }
end