Emberjs receiving and creating embedded records with emberfire

I'm trying to add embedded records into fire base with the ember fire adapter without any luck. So I've got a post and a comment model and I would like it to look like this in firebase:

{
  "posts": {
    "post_id_1": {
      "comments": {
        "comment_id_1": {
          "body": "This is a comment"
        }
      }
    }
  }
}

I've currently got this:

{
  "posts": {
    "post_id_1": {
      "comments": {
        "comment_id_1": true
      }
    }
  },
  "comments": {
    "comment_id_1": {
      "body": "This is a comment",
      "post": "post_id_1"
    }
  }
}

I've tired following the documentation Docs but can only get the Async working but the not the embedded relationship to work. I'm working with an existing fire base database so I can't change the data structure on the backend it needs to be in the embedded format.

Finally is this best of way of doing this or is there a better way of doing?

I've also tried doing this as suggested here Docs

let post = this.get('post');
let newComment = this.store.createRecord({
  body: 'My super fun embedded comment'
});

post.get('comments').then(function(comments) {
  comments.addObject(newComment);
  // The comment is automatically saved when we call save() on the parent:
  return post.save();
});

Error in console:

Error: Assertion Failed: Passing classes to store methods has been removed. Please pass a dasherized string instead of [object Object]