POSTing a @OneToMany sub-resource association in Spring Data REST

Assuming you already have discovered the post URI and thus the URI of the association resource (considered to be $association_uri in the following), it generally takes these steps:

1. Discover the collection resource managing comments:

    curl -X GET http://localhost:8080
   
    200 OK
    { _links : {
        comments : { href : "…" },
        posts :  { href : "…" }
      }
    }
   

2. Follow the comments link and POST your data to the resource:

    curl -X POST -H "Content-Type: application/json" $url 
    { … // your payload // … }
   
    201 Created
    Location: $comment_url
   

3. Assign the comment to the post by issuing a PUT to the association URI.

    curl -X PUT -H "Content-Type: text/uri-list" $association_url
    $comment_url
   
    204 No Content
   

Note, that in the last step, according to the specification of text/uri-list, you can submit multiple URIs identifying comments separated by a line break to assign multiple comments at once.

A few more notes on the general design decisions. A post/comments example is usually a great example for an aggregate, which means I’d avoid the back-reference from the Comment to the Post and also avoid the CommentRepository completely. If the comments don’t have a lifecycle on their own (which they usually don’t in an composition-style relationship) you rather get the comments rendered inline directly and the entire process of adding and removing comments can rather be dealt with by using JSON Patch. Spring Data REST has added support for that in the latest release candidate for the upcoming version 2.2.