How to POST a comment and other relational entities

Problem/Motivation

It's not clear how to POST a comment (and probably other entities having relations like book page, entity referenced entity).

Submitting a node by following the REST+HAL node POST on https://www.drupal.org/node/2098511 is running ok.

Trying to accomplish this for a comment fails with 422 Unprocessable Entity codewithout the build message by EntityResource::validate()

Adding throw new \RuntimeException($message); like

<?php
protected function validate(EntityInterface $entity) {
...
throw new \RuntimeException($message);
      throw new HttpException(422, $message);
    }
  }
?>

reveils in apache error log in my case shows

Uncaught PHP Exception RuntimeException: "Unprocessable Entity: validation failed.\nentity_id: This value should not be null.\n" at /.../core/modules/rest/src/Plugin/rest/resource/EntityResource.php line 214

What must I do now?

Proposed resolution

This needs documentation and debugging tools for REST client builders.

Remaining tasks

• Write documentation
• Move forward #1925618: Add documentation for REST link relations
• Should we throw an error or provide a JSON error struct as discussed in #1996130: REST views: not adding dependencies on Serializer providers and #1916302: Serve REST errors as application/api-problem+json OR application/vnd.error+json

Collateral issue:

• Is Symfony in err not using getMessage()? File a bug for Symfony?

User interface changes

API changes