<?php

/**
 * @file 
 * Allows two content types which nodereference each other to stay in sync:
 * When node A references node B, node B also gets a reference to node A.
 * Comes from http://drupal.org/node/120408 by chx.
 */

function backreference_nodeapi($node, $op) {
  switch ($op) {
    case 'insert': case 'update':
      $fields = content_types($node->type);
      foreach ($fields['fields'] as $field_name => $field) {
        if ($field['type'] == 'nodereference') {
          foreach (array_filter($field['referenceable_types']) as $related_type) {
            _backreference_insert($related_type, $field_name, $node);
          }
        }
      }
      break;
  }
}

function _backreference_insert($type, $key, $node) {
  $fields = content_types($type);
  foreach ($fields['fields'] as $field) {
    if ($field['type'] == 'nodereference' && $field['referenceable_types'][$node->type]) {
      $info = content_database_info($field);
      $table = $info['table'];
      $fieldname = $info['columns']['nid']['column'];
      $new_nids = array($node->nid);
      $placeholders = array();
      foreach ($node->$key as $item) {
        if ($item['nid']) {
          $new_nids[] = $item['nid'];
          $placeholders[] = '%d';
        }
      }
      $placeholders = $placeholders ? implode(', ', $placeholders) : '%d';
      // We pick the nodes that no longer point to us.
      $sql = 'FROM {'. $table .'} WHERE '. $fieldname .' = %d AND nid NOT IN ('. $placeholders .')';
      $result = db_query('SELECT nid, vid '. $sql, $new_nids);
      while ($existing_node = db_fetch_object($result)) {
        cache_clear_all('content:'. $existing_node->nid .':'. $existing_node->vid, 'cache_content');
      }
      // We delete the references that no longer point to us.
      db_query('DELETE '. $sql, $new_nids);
      unset($new_nids[0]);
      foreach ($new_nids as $nid) {
        $vid = db_result(db_query('SELECT vid FROM {node} WHERE nid = %d', $nid));
        // Skip existing references.
        if (!db_result(db_query('SELECT 1 FROM {'. $table .'} WHERE '. $fieldname .' = %d AND nid = %d AND vid = %d', $node->nid, $nid, $vid))) {
          // This is a new reference, so we need to insert it after other references.
          // This is race condition, yes, but I do not have anything better. Do we want one sequence per nid?
          // Remove any NULL values first.
          db_query('DELETE FROM {'. $table .'} WHERE ' . $fieldname .' IS NULL AND vid = %d', $vid);
          $delta = db_result(db_query('SELECT MAX(delta) + 1 FROM {'. $table .'} WHERE nid = %d', $nid));
          db_query('INSERT INTO {'. $table .'} (vid, delta, nid, '. $fieldname .') VALUES (%d, %d, %d, %d)', $vid, $delta, $nid, $node->nid);
          cache_clear_all('content:'. $nid .':'. $vid, 'cache_content');
        }
      }
    }
  } 
}