For others that have spent countless hours getting this to work and are struggling... in particular assigning the correct user to the comment - this is how I achieved the correct outcome.
The outline below is for usage via Postman and DHC (Restlec Client)... once working here then testing done and ready to apply to your app or however you are using this.
Set up
Set accept to hal+json
sample:
POST request
url: http://example.com/entity/comment?_format=hal_json
Authorization Type: Basic {then enter username and password in the corresponding fields)
Headers
Authorization: {this field should be populated from the step earlier where you entered your username and password. If it is not, then go back to the authorization tab and click update request}
Content-Type: application/hal+json
X-CSRF-Token: get your token via http://example.com/rest/session/token
Accept: application/hal+json
JSON formatted data to send to Drupal
{"_links":{
"http://example.local/rest/relation/comment/comment/uid":{"href": "http://example.local/user/2?_format=hal_json"},
"http://example.local/relation/comment/comment/entity_id":{"href":"http://example.local/node/2?_format=hal_json"},
"type":{"href":"http://example.local/rest/type/comment/comment"}
},
"entity_id":[{"target_id":"2"}],
"langcode":[{"value":"en"}],
"subject":[{"value":"Comment title"}],
"entity_type":[{"value":"node"}],
"comment_type":[{"target_id":"comment"}],
"field_name":[{"value":"comment"}],
"comment_body":[{"value":"Body text for the comment.",
"basic_html":"basic_html"}],
"_embedded": {
"http://example.local/rest/relation/comment/comment/entity_id": [{
"_links": {
"self": {"href": "http://example.local/node/25?_format=hal_json"},
"type": {"href": "http://example.local/rest/type/node/article"}
},
"uuid": [{"value": "8795cd5d-d223-4cad-9373-605c8ffac10f"}]
}],
"http://example.local/rest/relation/comment/comment/uid": [{
"_links": {
"self": {"href": "http://example.local/user/1?_format=hal_json"},
"type": {"href": "http://example.local/rest/type/user/user"}
},
"uuid": [{"value": "a8c95c05-4546-4c7b-def5-fed7a4c5beca"}]
}]
}
}
Errors
If you are applying the above and the following errors occur:
415 Unsupported Media Type - then most likely the Content-Type is set to application/json rather than Content-Type being set to application/hal+json
Change - Content-Type: application/hal+json
401 Forbidden "message": "Access denied on creating field 'uid'." - as noted above it is a permission issue. What exactly is this permission issue... this tripped me as authenticated users could already add comments directly to the site. However, with REST you also need to check 'Administer comments and comment settings' against the authenticated user (go to http://example.com/admin/people/permissions and check).
Change - Administer comments and comment settings in the permissions area
Assigning new comments against the wrong user - it's the _embedded uuid.
You need to check that you have the correct uuid for both the entity_id and the uid
- entity_id - "http://example.local/rest/relation/comment/comment/entity_id";
- uid - "http://example.local/rest/relation/comment/comment/uid"
Testing this, I found that you can change the uuid for the entity and even though the "http://example.local/relation/comment/comment/entity_id":{"href":"http://example.local/node/2?_format=hal_json"}, is set correctly, the comment will be assigned to entity of the uuid in the _embedded area. This also applies to the "http://example.local/rest/relation/comment/comment/uid"
Simply the request body
You can actually simplify the body of the request from what is above to:
{"_links":{
"type":{"href":"http://example.local/rest/type/comment/comment"}
},
"entity_id":[{"target_id":"2"}],
"langcode":[{"value":"en"}],
"subject":[{"value":"Comment title"}],
"entity_type":[{"value":"node"}],
"comment_type":[{"target_id":"comment"}],
"field_name":[{"value":"comment"}],
"comment_body":[{"value":"Body text for the comment.",
"basic_html":"basic_html"}],
"_embedded": {
"http://example.local/rest/relation/comment/comment/entity_id": [{
"uuid": [{"value": "8795cd5d-d223-4cad-9373-605c8ffac10f"}]
}],
"http://example.local/rest/relation/comment/comment/uid": [{
"uuid": [{"value": "a8c95c05-4546-4c7b-def5-fed7a4c5beca"}]
}]
}
}