Get a collection of sub-resources at once with JSON-LD and Hydra - rest

Question

In the RESTful Web API book, the authors advise to expose a profile and use a content type which acknowledges link relations. JSON-LD extended by Hydra seem to match these requirements, and I want to use them in the design of my new API.
I am currently stuck with a performance issue. Let say that I have an online bike store, and I want to retrieve information about the wheels of a given bike.
With the Hydra specification, it seems to me that I need to send 2 requests to get the details about the wheels.
The first request is toward the bike itself:
GET /mybike HTTP/1.1
Host: wowbike.com
The response contains a Hydra::Link to the collection of wheels:
HTTP/1.1 200 OK
Content-Type: application/ld+json
{
"#context" :
{
"Bike": "/contexts/vocab#Bike"
},
"#id" : "/mybike",
"#type" : "Bike",
"size" : "L",
"wheels" : "/mybike/wheels" // "wheels" is a "hydra:Link"
}
Now I can send a second request to the wheels resource to get the details:
GET /mybike/wheels HTTP/1.1
Host: wowbike.com
HTTP/1.1 200 OK
Content-Type: application/ld+json
{
"#context":
{
"Collection": "http://www.w3.org/ns/hydra/core#Collection",
"Wheel" : "/contexts/vocab#Wheel"
},
"#type" : "Collection",
"#id" : "/mybike/wheels",
"member" :
[
{
"#id" : "/mybike/wheels/firstwheel",
"#type" : "Wheel",
"color" : "blue"
},
{
"#id" : "/mybike/wheels/secondwheel",
"#type" : "Wheel",
"color" : "white"
}
]
}
Is it valid to send a single request and get a response such as the one below?
GET /mybike HTTP/1.1
Host: wowbike.com
HTTP/1.1 200 OK
Content-Type: application/ld+json
{
"#context" :
{
"Collection": "http://www.w3.org/ns/hydra/core#Collection",
"Bike" : "/contexts/vocab#Bike",
"Wheel" : "/contexts/vocab#Wheel"
},
"#id" : "/mybike",
"#type" : "Bike",
"size" : "L",
"wheels" :
{
"#id" : "/mybike/wheels",
"#type" : "Link",
"member":
[
{
"#id" : "/mybike/wheels/firstwheel",
"#type" : "Wheel",
"color" : "blue"
},
{
"#id" : "/mybike/wheels/secondwheel",
"#type" : "Wheel",
"color" : "white"
}
]
}
}

Answer 1

Great to see that you consider using JSON-LD and Hydra. Of course it is possible to get all the data in a single response. You don't have to change the type of the collection from Collection to Link though. Also, you might want to tweak your context a bit. Summed up, your response would look somewhat like this:
{
"#context": [
"http://www.w3.org/ns/hydra/context.jsonld",
{ "#vocab": "/contexts/vocab#" }
],
"#id": "/mybike",
"#type": "Bike",
"size": "L",
"wheels": {
"#id" : "/mybike/wheels",
"#type" : "Collection",
"member": [
{
"#id" : "/mybike/wheels/firstwheel",
"#type" : "Wheel",
"color" : "blue"
},
{
"#id" : "/mybike/wheels/secondwheel",
"#type" : "Wheel",
"color" : "white"
}
]
}
}
I'm importing Hydra's context here and then overlaying a default vocabulary which means that everything that isn't already defined in Hydra's context is expanded by appending it to /contexts/vocab#. So, Bike for instance will be expanded to /contexts/vocab#Bike.
Btw. there's a W3C Community Groupw working on Hydra which you should join if you are using it. We also have a mailing list on which all your questions will be answered.
The instructions to join the group can be found at http://www.hydra-cg.com/#community