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1. drupal
   1. drupal6
   2. drupal7

Check that the book has not been changed while using the form.

See also

book_admin_edit()

Code

modules/book/book.admin.inc, line 88

<?php
function book_admin_edit_validate($form, &$form_state) {
  if ($form_state['values']['tree_hash'] != $form_state['values']['tree_current_hash']) {
    form_set_error('', t('This book has been modified by another user, the changes could not be saved.'));
    $form_state['rebuild'] = TRUE;
  }
}
?>