title: Kohana ORM: Validate the $_belongs_to relationship exists
body: <p>I'm trying to get some validation setup for one of my ORM models.</p>
  
  <p>I have 2 tables: parent and children. In the children table, there is a column called 'parent' whose value is the primary ID of a row in the parent table.</p>
  
  <p>What I'm trying to do, is create a validation rule that checks the parent ID specified actually exists in the parent table.</p>
  
  <p>Is there an easy way to do this? </p>