How do I programmatically set a flag's custom entity reference field when programmatically setting the flag?

Question

Some background: I'm new to Drupal 8/9 module building. I need a particular content type's nodes to be automatically flagged. I have this working using the code below. However, the flag involved also has a custom field I need to set.

• It's an entity reference field (to another node)
• I want to set the flag's custom entity reference field to be the same value as an entity reference field on the flagged node.

(This may seem like a strange thing to do. There may be a better way to accomplish the UX goals. I still want to know how to do it.)

'rf_tweaks' is the module machine name, 'research_note' is the node content type, and 'research_item' is the name of the flag.

use Drupal\node\Entity\Node;

function rf_tweaks_node_insert(Node $node) {
  if ($node->bundle() == 'research_note'){
 $flag = \Drupal::entityTypeManager()->getStorage('flag')->load('research_item');
 \Drupal::service('flag')->flag($flag, $node);
}
}

There's some great info at the flag api page, but the methods are different enough that it's not clear to me how to adapt them to this situation.

Answer 1

The method in your last line of code returns the "flagging" entity, which represents the entity/flag relationship. That's the thing you need to set the field on. e.g.

$flag = \Drupal::entityTypeManager()->getStorage('flag')->load('research_item');
$flagging = \Drupal::service('flag')->flag($flag, $node);

$flagging->field_foo->target_id = $node->field_bar->target_id;
$flagging->save();

That does involve the flagging entity being saved twice, so if performance ever becomes an issue you might need to create a Flagging entity manually and save it to avoid the double-hit.