In REST how do you retrieve resources identified by 2 attributes? - web-services

Question

In REST, if you want to retrieve a resource, you use something like:
http://www.blabla.com/api/movies/1234
where 1234 is the unique identifier of the movie in the database.
The question I have how do you retrieve entities from the database that are identified by 2 attributes?
Maybe something like:
http://www.blabla.com/api/grade/1234/764334532 (identified by the combination of user and exam id)
Or perhaps I have to model the database or resources in other way, like adding an id to grades in the database.
What do you think? Thanks.

Answer 1

If your exam id is globally unique you can just use that in the URI. no need to reference the user:
http://www.blabla.com/api/exams/123
if the exam id is only locally unique to a user, then you need to include both, as follows:
http://www.blabla.com/api/users/456/exams/123

Answer 2

You need to take care that your ressources are addressed uniquely. Below there is an example (JAX-RS) of a customer service which enables users to insert and retrieve recensions to or from a specific product.
/**
* Abstract service customer. Contains methods for retrieving and persisting
* data in database.
*/
#Path("/customers")
public interface ServiceCustomer extends Service<Customer, Integer> {
/**
* Adds a customers {#link Recension} to a product
*
* #param recension
* The new created {#link Recension} by the customer
* #param customerId
* Identifier of the customer as Integer who has created the
* recension
* #param productId
* Identifier of the product as Integer which should be
* associated with the new created recension
* #return {#link Response}
*/
#POST
#Path("{id}/productId/{prodId}/recension")
#Consumes({ MediaType.APPLICATION_XML, MediaType.APPLICATION_JSON })
public Response insertRecension(Recension recension,
#PathParam("id") Integer customerId,
#PathParam("prodId") Integer productId);
/**
* Retrieves all customers {#link Recension}(s)
*
* #param customerId
* Identifier of the {#link Customer} as int
* #return {#link Response}, containing a {#link List} of all created
* {#link Recension}(s) by the customer
*/
#GET
#Produces({ MediaType.APPLICATION_XML, MediaType.APPLICATION_JSON })
#Path("{id}/recension")
public Response getRecensions(#PathParam("id") int customerId);
/**
* Retrieves a single {#link Recension} of a {#link Product} which was
* created by a specific {#link Customer}
*
* #param productID
* Identifier of the {#link Product}
* #param customerID
* Identifier of the {#link Customer}
* #return {#link Response}, which contains the {#link Recension} of the
* {#link Product} created by the user. Assuming the user has
* created a recension.
*/
#GET
#Produces({ MediaType.APPLICATION_XML, MediaType.APPLICATION_JSON })
#Path("{id}/productId/{prodId}/recension")
public Response getRecensionOfProductByCustomer(
#PathParam("prodId") int productID, #PathParam("id") int customerID);
}
The first method insertRecension is used by a customer to create a recension to a specific product. Hereby you need to define two identifiers, the id of the customer who is creating the recension for the product with the specific id.
#Path("{id}/productId/{prodId}/recension")
The second method getRecensions is used by a customer to retrieve all his created recensions. For this purpose, the rest- endpoint just needs to know a single identifier, the customers id.
#Path("{id}/recension")
The last method getRecensionOfProductByCustomer retrieves the recension of a specific product, created by a specific user. Notice, that this rest- endpoint takes two identifiers too!
#Path("{id}/productId/{prodId}/recension")

Answer 3

It's hard to say without more information, but I'd consider a lightweight /grades URI, where each grade is the intersection of an exam and a user:
GET /grades?exam=3&userId=bob
{
"id": 343,
"self": "/grades/343",
"exam": "/exams/3",
"user": "/users/bob",
"value": 88
}
You can then easily find all the grades for an exam or a user by only specifying one of the two query parameters.