How to Create Comments and Associate Them with a Post in Spring Data REST

How do I create a Comment resource and associate it with a Post in a Spring Data REST application without posting directly to the /comments endpoint?

{
    "author":"commentAuthor",
    "content":"This is a comment"
}

In a Spring Data REST application, managing relationships between entities, such as `Post` and `Comment`, can be done using RESTful practices. This guide explains how to post a `Comment` as a sub-resource of a `Post` properly and troubleshoot common issues, such as HTTP 405 errors.

POST http://localhost:8080/posts/1/comments
Content-Type: application/json

{
    "author":"commentAuthor",
    "content":"This is a comment"
} 

// Expected response:
{
    "id": 1,
    "author": "commentAuthor",
    "content": "This is a comment",
    "post": {
        "href": "http://localhost:8080/posts/1"
    }
}

Causes

• The `POST /posts/{id}/comments` endpoint may not be correctly set up in your application.
• Spring Data REST does not automatically handle sub-resource creation unless properly configured.
• Your Comment class may not have the appropriate mapping or saving logic in place.

Solutions

• Ensure that your Comment resource is annotated with the `@RestResource` to expose it correctly as a sub-resource.
• When creating a comment, POST to the endpoint `/posts/{id}/comments` using the correct JSON format, which only includes comment properties (not the post). This associates the comment with the specified post.
• Example request body for adding a comment: { "author":"commentAuthor", "content":"This is a comment" }

Mistake: Attempting to include the `post` object in the comment payload.

Solution: When posting a new comment, only include data that belongs to the Comment entity like `author` and `content`. The association to Post is managed by the endpoint.

Mistake: Using the wrong HTTP method or incorrect URL for posting comments.

Solution: Make sure you are using POST to the correct endpoint format: `/posts/{id}/comments`.