There was a great question on Drupal developers mailing list the other day—one to which I've "rediscovered" the solution to a few times—so I wanted to make sure that everyone was aware of it.
The basic question is:
When a node is being saved, how can you see what values have changed?
The short answer is:
Use the 'presave' operation to load a copy of the node before it's saved, stick it back into the node object, and in your 'update' operation code compare the "before" and "after" versions:
<?php
/**
* Implementation of hook_nodeapi().
*/
function example_nodeapi(&$node, $op, $a3, $a4) {
// We want to compare nodes with their previous versions. Ignore new
// nodes with no nid since there's no previous version to load.
if ($op == 'presave' && !empty($node->nid)) {
// We don't want to collide with values set by other modules so we'll
// use the module name as a prefix and a long name to be save.
$node->example_presave_node = node_load($node->nid);
}
elseif ($op == 'update') {
// On update we pull the previous version out of the node and compare
// it to the newly saved one.
$presave = $node->example_presave_node;
// Pretend we're comparing a single value CCK number field here.
$field_name = 'field_example';
if ($node->$field_name != $presave->$field_name) {
drupal_set_message(
t("The node's value changed from %previous to %current.", array(
'%previous' => $presave->$field_name[0]['value'],
'%current' => $node->$field_name[0]['value'],
))
);
}
}
}
?>
You can also use the Rules
You can also use the Rules module, to compare two fields of the unchanged content Vs updated content. No coding required, it has a UI and is exportable.
or...
Or, if you're using Forms API, there's a copy of the original node in $form['#node']
I feel like I ran into issues
I feel like I ran into issues where modules would use that as their working copy and alter it.