factories-1.rb

factory :user do
  email "[email protected]"
  password "secret"
  
  trait :with_comments do
    after(:create) do |user, evaluator|
      # FactoryGirl.create_list(factory, number_of_items, factory_attrs)
      FactoryGirl.create_list(:comment, 1, user: user)
    end
  end
end

factory :comment do
  text "This is a comment."
end

factories-2.rb

class User < ActiveRecord::Base
  has_many :comments  
end

class Comment < ActiveRecord::Base
  belongs_to :user
end