How can I use UUID in a Views relationship?
I figured out how to do this. It doesn't require a custom relationship handler. In MYMODULE.views.inc:

/**
 * Implements hook_views_data_alter()
 */
function MYMODULE_views_data_alter(&amp;$data) {
  // Add a relationship between the field_subscription_uuid field and node UUIDs
  $data['field_data_field_subscription_uuid']['field_subscription_uuid_value']['relationship'] = array(
    'title' =&gt; t('UUID'),
    'help' =&gt; t('Relationship for field_subscription_uuid to node UUID.'),
    'handler' =&gt; 'views_handler_relationship',
    'base' =&gt; 'node',
    'base field' =&gt; 'uuid',
    'label' =&gt; t('UUID'),
  );
}