Posting data with relationships to a REST API - angularjs

Question

I am creating an API and I need to create a Post which is defined as:
{
"Title": "Blog first post",
"Body": "Some body text for the post",
"PublishedAt": "2016-02-08",
"Category": {
"Id": "20",
"Name": "Travel"
},
"Tags": [
{ "Id": "12", "Name": "Vacation" },
{ "Id": "18", "Name": "Beach" }
]
}
What would you post to the API? The following?
{
"Title": "Blog first post",
"Body": "Some body text for the post",
"PublishedAt": "2016-02-08",
"Category": {
"Id": "20",
},
"Tags": [
{ "Id": "12" },
{ "Id": "18" }
]
}
Or would you do this by steps as follows:
POST /category/20/posts
POST /posts/recently-created-post-id/tags >> Add tags one by one?
But what to do when the entire data is ready to post?
What is the standard for this situations in a REST API?

Answer 1

Depends on how you want to present your API and how thorough you want it to be. I would do the following given your models.
I would have one endpoint at POST /posts and would send the entire body of the post model.
I would also have one endpoint at POST /posts/{id}/tags to allow tags to be submitted after the post is created. So a call to POST /posts/{id}/tags would receive a body of:
{ "Id" : 12 }
For simplicity sake, many people would recommend that you stick to only one level which would mean you would create an endpoint for PUT /posts/{id} or PATCH /posts/{id} and submit your new tags through those endpoints. A PUT request would submit the full body you used in the POST along with any new tags. The purpose of a PUT is to replace the existing post model entirely with the new body. A PATCH request would submit a body that adhere's to a REST PATCH endpoint which allows modifications to an object model without submitting the entire model.
If you were going with the multi-level model, you would need to support PUT and PATCH requests at the endpoint PUT /posts/{id}/tags/{tagid} and PATCH /posts/{id}/tags/{tagid}. Since your tags only contain a reference id, PUT and PATCH does not make sense because all you want to do is add or delete tags.
I assume you would also need to handle tag deletions. With the one level endpoint approach, you would submit the entire post model without the tag you wanted to remove via a PUT request to /posts/{id}. For example, if you wanted to delete tag 12 your :
{
"Title": "Blog first post",
"Body": "Some body text for the post",
"PublishedAt": "2016-02-08",
"Category": {
"Id": "20",
},
"Tags": [
{ "Id": "18" }
]
}
If you wanted to go with the multi-level approach, you would issue a DELETE request to /posts/{id}/tags/12.

Answer 2

There's no hard and fast rule for these situations in REST.
If we assume that the client wants to create everything in one go and wait the least amount of time possible for a response, common sense would suggest keeping everything in one request to avoid (a)synchronicity problems between the main POST and POSTs for the tags.