Not possible to remove all terms from a node using node_save().

Let N be a node associated with terms given by tids t1 and t2. The internal taxonomy array for N may look like: array (t1, t2). Associating N with a third term given by tid t3 is a simple process of loading the node, adding t3 to the internal taxonomy array, and saving the node. Therefore, you would expect that the process of disassociating N from all terms might be: load the node, set it's internal taxonomy array to the empty array, and save the node. Unfortunately, this seemingly sane methodology is flawed due to a poor assumption made in file `taxonomy.module'. Consider the following snippet:

/**
 * Implementation of hook_nodeapi().
 */
function taxonomy_nodeapi($node, $op, $arg = 0) {
  switch ($op) {

    ...

    case 'update':
      if (!empty($node->taxonomy)) {
        taxonomy_node_save($node, $node->taxonomy);
      }
      break;

    ...

  }
}

The culprit lies on line 1212: if (!empty($node->taxonomy)) {. So we only save/modify the terms if the array is not empty, hence if the array is empty nothing happens. My proposed fix would be to replace line 1212 with:

if (isset($node->taxonomy)) {

which will cause the taxonomy module to invoke taxonomy_node_save() even when the internal taxonomy array is empty.

Tom