sign_up.rb

def save
  ActiveRecord::Base.transaction do
    user = User.save(username: username) # user doesn't have to be instance var @user
    add_errors(user.errors) unless user.persisted?
    tweet = Tweet.save(body: first_tweet, user_id: user.id)
    add_errors(tweet.errors) unless tweet.persisted?
    user.persisted? && tweet.persisted?
  end
end