Generic diff function to diff node field current and original valeu on presave

gistfile1.txt

function _my_module_diff_field_values($node, $field_name) {
  $array_a = array();
  $array_b = array();

  if(!empty($node->{$field_name})) {
    foreach($node->{$field_name}[LANGUAGE_NONE] as $key => $value) {

      if(isset( $value['nid'])) {
        $array_a[] = $value['nid'];
      }   
      // Try value
      elseif(isset( $value['value'])) {
        $array_a[] = $value['value'];
      }   
    }   
  }

  if(!empty($node->original->{$field_name})) {
    foreach($node->original->{$field_name}[LANGUAGE_NONE] as $key => $value) {
      // Try nid
      if(isset( $value['nid'])) {
        $array_b[] = $value['nid'];
      }   
      // Try value
      elseif(isset( $value['value'])) {
        $array_b[] = $value['value'];
      }   

    }   
  }

  $diff = array_diff($array_a, $array_b);
  $rev_diff = array_diff($array_b, $array_a);
  if(!empty($diff)) {
    return TRUE;
  } elseif(!empty($rev_diff)) {
    return TRUE;
  } else {
    return FALSE;
  }
}