title: Get children of a collection of parents with ActiveRecord
body: <p>I have a Category model and Post model with a one-to-many relationship.</p>
  
  <p>I'd like to have all posts for a certain set of categories.</p>
  
  <p>I want the result to be an ActiveRecord object to be able to do further queries.</p>
  
  <p>Right now I'm using <code>.map</code> like so</p>
  
  <pre><code>categories.map{|c| c.posts.order(position: :asc)}
  </code></pre>