How to modify field value in other nodes after node update?

Question

After node update I need to modify a field value in several connected nodes.
As I know, I cannot use hook_entity_update() since $node->save() forms infinite recursion.

How can I modify node fields after a node has been updated? (besides hook_cron)

Answer 1

Use hook_form_alter or hook_form_FORM_ID_alter

Add a custom validation that runs after all validation has passed

$form['actions']['submit']['#submit'][] = 'mymodule_update_referenced_nodes';

Then in that function

function mymodule_update_referenced_nodes(&$form, \Drupal\Core\Form\FormStateInterface $form_state){

  $field_other = $form_state->getValue('field_other')[0]['value'];

  $references = $node->get('field_referenced')->referencedEntities();
 
  foreach ($references as $referenced_node) {
    $referenced_node->field_other->value = $field_other;
    $referenced_node->save();
  }

}