title: Equivalent of find_each for foo_ids?
body: <p>Given this model:</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    has_many :things
  end
  </code></pre>
  
  <p>Then we can do this::</p>
  
  <pre><code>@user = User.find(123)
  @user.things.find_each{ |t| print t.name }
  @user.thing_ids.each{ |id| print id }
  </code></pre>
  
  <p>There are a large number of <code>@user.things</code> and I want to iterate through only their ids in batches, like with <code>find_each</code>. Is there a handy way to do this?</p>
  
  <p>The goal is to:</p>
  
  <ul>
  <li>not load the entire <code>thing_ids</code> array into memory at once</li>
  <li>still only load arrays of <code>thing_ids</code>, and not instantiate a <code>Thing</code> for each id</li>
  </ul>