How to programmatically append fields to node object when cloning with createDuplicate()

I am trying to setup a custom module which copies values from a node whose content type is A to a node whose content type is B. For the most part, the content types have the same fields, but B has a few additional fields.

A

• Title (title)
• Body (body)
• Date (field_date_agenda)

B

• Title (title)
• Body (body)
• Date (field_date_agenda)
• Number of items (field_int_how_many)
• Reference to A (field_noderef_nta)

With $cloned_node = $node->createDuplicate() I am able to copy all the fields from A, as well as being able to change the content type to B.
Since A doesn't have the extra fields attached to it, createDuplicate() won't copy them.

Now, what I would like to do is to programmatically add the other fields (which I will populate with some custom values) to $cloned_node. When these actions are completed, $cloned_node->save() will be used to finalize the cloning process.

Simply adding the value via $cloned_node->set("field_int_how_many", $value) doesn't seem to do the trick: A PHP error is thrown.

InvalidArgumentException: Field field_int_how_many is unknown. in Drupal\Core\Entity\ContentEntityBase->getTranslatedField() (line 587 of /home/username/domains/domainname.com/public_html/devsite/web/core/lib/Drupal/Core/Entity/ContentEntityBase.php).

Does anyone have an idea how to add those extra fields to the cloned node?

If the content type of $node is A, the content type of the node returned by $node->createDuplicate() is still A. That's why $node->createDuplicate()->set("field_int_how_many", $value) returns an error saying that field_int_how_many is an unknown field: Content type A doesn't have that field.

What you are trying to obtain isn't a clone of $node, but to copy the values from a node whose content type is A to a node whose content type is B.
For that, you need to create a new node, passing the values of the existing node field as values for the new node fields.

$new_node = Node::create(
  'type' => 'B',
  'title' => $node->title,
  'body' => $node->body,
  'field_date_agenda' => $node->field_data_agenda,
  'field_int_how_many' => /* the value to assign to the field */
);