title: Mongoid Search Gem does not update keywords on association
body: <p>I have an e-commerce application where I am using mongoid search to search through an order and its user association. Here are my models.</p>
  <pre><code>order.rb
  
  class Order
     include Mongoid::Document
     include Mongoid::Search
  
     belongs_to :user
  
     search_in user: %i[first_name last_name email_address]
  end
  
  user.rb
  
  class User
     include Mongoid::Document
     include Mongoid::Search
  
     has_many :orders
  
     search_in :email_address, :first_name, :last_name
  end
  </code></pre>
  <p>Basically the end goal of using search is to find a User's order by a users first name, last name, and email address. On initial pass, I can create an user and order and the <code>keywords</code> that mongoid uses to search get populated.</p>
  <p>However, if I update a user's first name, last name, or email address, the keywords for <code>Order</code> doesnt get updated. Hence the search breaks and the accuracy decreases.</p>
  <p>According to the documentation, I can do the following command <code>rake mongoid_search:index</code> and the keywords will be populated with the correct ones once more.</p>
  <p>However, in production I can't run this command everytime something changes. I need it to update automatically. Is there something that I'm doing wrong where if I update assocciated objects the keywords won't change?</p>