How to programmatically get the difference between two entities of same content type?

Question

I need to implement a function that checks if there has been a change from the previous existing node. If yes, send the updated changes.

I understand that Drupal has revision system and through diff we can see the difference between two revisions.

I want similar to that, however as an API call or a logic which can facilitate getting the difference.

Answer 1

Either look at:

• https://www.drupal.org/project/diff
• https://www.drupal.org/project/html_diff

To easily "see" if changes exist. Or look at Migrates module track_changes implementation and implement it in code between revisions. All track_changes does is hash the fields of a content type together to get a unique ID. Different revisions of a node with changes between the body field will of course hash to a different value. Be sure to exclude node fields such as NID, vid, updated_at and other fields that (may) chnage between revisions.

Sample hash code from class MigrateSource:

// When tracking changed data, We want to quietly skip (rather than
// "ignore") rows with changes. The caller needs to make that decision,
// so we need to provide them with the necessary information (before and
// after hashes).
if ($this->trackChanges) {
    $unhashed_row = clone ($row);
    // Remove all map data, otherwise we'll have a false positive on the
    // second import (attempt) on a row.
    foreach ($unhashed_row as $field => $data) {
      if (strpos($field, 'migrate_map_') === 0) {
        unset($unhashed_row->$field);
      }
    }
    $row->migrate_map_original_hash = isset($row->migrate_map_hash) ?
      $row->migrate_map_hash : '';
    $row->migrate_map_hash = $this->hash($unhashed_row);
  }
  else {
    $row->migrate_map_hash = '';
  }

Answer 2

Amit's answer is super helpful, thanks for that. But there is one major error:

$diff = array_diff($new_value, $old_value);

PHP's array_diff will only tell you about values that are in the first param that are not in the second param. So, if we are updating a node and we delete (instead of add) a new value i na field, array_diff will not notice this and the node will not be marked by this code as changed (assuming that is the only change).

Instead, that line should be changed to:

$diff = array_merge(array_diff($new_value, $old_value), array_diff($old_value, $new_value));

I tested this locally, successfully.

I picked up that array_merge tidbit from https://stackoverflow.com/questions/10077840/difference-between-two-arrays

I don't have enough rep to comment on the post, but I can edit it, subject to moderation, so I will do that.

Answer 3

I wanted to know if there were any changes whenever a node gets updated. If yes, trigger the mail. The solution below works well for me (It could be improved though). I didn't try to get the exact difference, which however can be found using field_get_items() api.

$entity_old = entity_metadata_wrapper('node', $node->original);
$entity_new = entity_metadata_wrapper('node', $node);
// Fields comprising of field names array, whose changes you want to ignore
$ignore_fields_list = ignore_fields_list();
if(!is_object($entity_old)) {
  return;
}

$data = field_info_instances('node' ,$entity_new->getBundle());
$fields_list = array_keys($data);
$diff_list =  array();
foreach ($fields_list as $key => $field_name ) {
// Ignore if field name belong to ignore list
if(in_array($field_name, $ignore_fields_list )) {
  continue;
}

$old_value = $entity_old->{$field_name}->raw();
$new_value = $entity_new->{$field_name}->raw();
// If both are empty - ignore
if(empty($old_value) && empty($new_value)) {
  continue;
}
 // If one of the value is not empty compared to the other
if( (empty($old_value) && !empty($new_value) ) || (empty($new_value) && !empty($old_value))){
  // You can use field_get_items() to get the changed value.
  $diff_list[$field_name] = $data[$field_name]['label'];
  continue;
}

if(!is_array($new_value) && $new_value != $old_value) {
  $diff_list[$field_name] = $data[$field_name]['label'];
  continue;
}

if(is_array($new_value) && is_array($old_value)) {
  $diff = array_merge(array_diff($new_value, $old_value), array_diff($old_value, $new_value));
  if(!empty($diff)) {
    $diff_list[$field_name] = $data[$field_name]['label'];
  }
}
}

return $diff_list;