Extending a dedupe rule to change a condition to stipulate a specific value, and to add a condition on a subtype custom field

gistfile1.php

/**
 * Implementation of hook_civicrm_dupeQuery
 *
 * @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_dupeQuery
 */
function extendeddedupe_civicrm_dupeQuery($obj, $type, &$query) {
  if ($obj->id != STUDENT_DEDUPE_RULE_ID || $type != 'table') {
    return;
  }
  watchdog(
    'Extended Dedupe',
    'dupeQuery before: obj :obj, type :type, query :query',
    array(
      ':obj'   => print_r($obj, TRUE),
      ':type'  => $type,
      ':query' => print_r($query, TRUE),
    ),
    WATCHDOG_DEBUG
  );

  // Change the contact subtype condition (if present) to say that they must be Students
  $subtypeQuery = CRM_Utils_Array::value('civicrm_contact.contact_sub_type.10', $query);
  if (!empty($subtypeQuery)) {
    $subtypeQuery = str_replace(
      "contact_sub_type IS NOT NULL",
      "contact_sub_type LIKE '%Student%'",
      $subtypeQuery
    );
    $query['civicrm_contact.contact_sub_type.10'] = $subtypeQuery;
  }

  // Add a condition that says their schools must match
  $schoolQuery = "
    SELECT t1.entity_id id1,
           t2.entity_id id2,
           10           weight
      FROM civicrm_value_contact_reference_9 t1
      JOIN civicrm_value_contact_reference_9 t2
     USING (contact_reference_21)
     WHERE t1.entity_id < t2.entity_id
       AND t1.contact_reference_21 IS NOT NULL
  ";

  // Was a group specified for this execution of the rule?
  if (!empty($obj->contactIds)) {
    foreach ($obj->contactIds as $cid) {
      CRM_Utils_Type::validate($cid, 'Positive');
    }
    $implodedIds = implode(',', $obj->contactIds);
    $schoolQuery .= "
      AND (t1.entity_id IN ($implodedIds)
       OR  t2.entity_id IN ($implodedIds))
    ";
  }
  $query['civicrm_value_contact_reference_9.contact_reference_21.10'] = $schoolQuery;

  watchdog(
    'Extended Dedupe',
    'dupeQuery after: obj :obj, type :type, query :query',
    array(
      ':obj'   => print_r($obj, TRUE),
      ':type'  => $type,
      ':query' => print_r($query, TRUE),
    ),
    WATCHDOG_DEBUG
  );
}