If you are like me you probably have searched for clues to and found little... particularly when confronted by the errors as you unravel how this works.  Okay a slight exaggeration, there are snipets of info that people have solved this annoying issue in  making comments work in Drupal 8 REST services. The lack of collective information in resolving issues has meant hours of debugging and working through core code to understand what is actually happening.  If you want to...

You have been working hard on getting comments being added (POST) through REST in Drupal 8.  However, now you are seeing the following Assigning the wrong user against a new comment - and most likely the new comment is being assigned to the site admin.   The error here is related to the _embedded uuid. What to do You need to check that you have the correct uuid for both the entity_id and the uid entity_id -...