RESTful API desgin: retrieve number of items in category - rest

Question

I want to create a REST API with items in categories and list all categories alongside its number of items.
Schemas:
Category {id, name}
Item {id, name, categoryId}
Endpoints:
GET /categories/list
GET /categories/<id>
PUT /categories/<id>
GET /items/list[?category=<categoryId>]
GET /items/<id>
To update a category I take what I get from GET /categories/<id>, modify the JSON object and PUT it back.
So far so good.
My question is if there are one ore more best practices to retrieve the item count?
I can think of a few ways to do this:
Fire a GET /items/list?category=<categoryId> for each category, counting the resulting items.
Taking the count from a X-total-count or content range header or a total_count field returned from the endpoint will avoid having to actually load all items.
Add an item_count field to the resulting category JSON objects.
How should this read only field be handled for PUTs? Make the backend ignore it? Manually unset it?
Create a dedicated endpoint /categories/item_counts that returns a list of categories with each number of items.
I like option number 2 (e.g. the wordpress API does it this way) because it does not need extra requests. But I really dislike the idea of having a different object structure for the GET and PUT requests.

Answer 1

REST is really about representation of objects. Category doesn't have a count as it's a single object. Item doesn't have a count either for the same reason. Count is more like RPC where you tell the service to compute something.
GET /items/list[?category=<categoryId>]
is like RPC, passing the category parameter to the list method. Staying in that idiom, you could "chain" the methods to get the total count of items in the specified category:
GET /items/list/count[?category=<categoryId>]
although I'd use path parameters instead:
GET /items/list/<category_id>/count
but list is implied so you could remove it:
GET /items/<category_id>/count
It's straying a bit from "pure" REST but it keeps your actual REST objects clean, as you say, keeping total_count our of your Category objects.
I'm assuming you sometimes need the count but not all the Items otherwise you wouldn't need to ask the API for the count, you'd just count them yourself in the client. That suggests another option:
GET /categories/<id>/count
{
"total_count": 10
}
which fits better with the use case of finding out how many Items are in a Category.