Delete objects from tables considering joins

This is another follow up of: #847682: Allow JOINS in ctools exports

CTools use the key from the schema to delete items from a table.

Now, the keys can be taken from another table. This make not possible to delete this kind of objects.

Here is an initial code for ctools_export_crud_delete()

function ctools_export_crud_delete($table, $object) {
  $schema = ctools_export_get_schema($table);
  $export = $schema['export'];

  if (!empty($export['delete callback']) && function_exists($export['delete callback'])) {
    return $export['delete callback']($object);
  }
  else {
    // If we were sent an object, get the export key from it. Otherwise
    // assume we were sent the export key.
    $value = is_object($object) ? $object->{$export['key']} : $object;
    $join_query = '';
    if (!empty($export['key in table']) && $export['key in table'] != $table) {
      foreach ($schema['join'] as $join) {
        if ($join['table'] == $export['key in table']) {
          $join_query = ", {$join['table']} t__1";
          $join_where = "AND t__0.{$join['left_key']} = t__1.{$join['right_key']}";
          if (!empty($join['extras'])) {
            $join_where .= ' ' . $join['extras'];
          }
          break;
        }
      }
    }
    db_query("DELETE t__0 FROM {$table} t__0 $join_query WHERE " . $export['key'] . " = '%s' $join_where", $value);
  }
}

I don0't know if this works fine in PostgreSQL, and if we should enabled to delete data for the tables on the inner joins. In my opinion, maybe we could call to another function to take care about those data.

More references about deleting tables using inner joins: http://dev.mysql.com/doc/refman/5.1/en/delete.html