title: Rails has_many, find only those with children
body: <p>My "products" table has_many :registered_products.</p>
  
  <p>I want to use something like </p>
  
  <pre><code>products.find(:has_registered_products) 
  </code></pre>
  
  <p>where that will return only the products that also have an entry in the registered_products table.  How could I achieve this?</p>