REST API Design with unidirectional relationships

Question

I am trying to build a REST API and am kind of at a loss as how to represent a many to 1 unidirectional relationships.

I have a 4 components that represents the 4 parts of a car (Engine, transmission, wheels, body).

Now obviously the component entities don't require a car entity, only the car cares about the components.

What i'm wondering is the best way to handle a REST input of the car resource.

1) Should I require a fully completed model to be submitted to the service (eg include a transmission with the full tranmission entity)?

2) Adjust the data coming in to only accept the ID of the required entity (eg engine_id, transmission_id)?

I'm a little confused on this and can't really find a good example after googling around.

public class Engine
{
    private Integer id;
    // Engine specific stuff
}

public class Transmission
{
    private Integer id;
    // Transmission specific stuff
}

public class Wheels
{
    private Integer id;
    // Wheels specific stuff
}

public class Body
{
    private Integer id;
    // Body specific stuff
}


public class Car
{
    @Id
    private Integer id;

    @ManyToOne
    @JoinColumn(name="engine_id")
    private Engine engine;

    @ManyToOne
    @JoinColumn(name="transmission_id")
    private Transmission transmission;

    @ManyToOne
    @JoinColumn(name="wheels_id")
    private Wheels wheels;

    @ManyToOne
    @JoinColumn(name="body_id")
    private Body body;
}

Answer 1

1) No. Cars obviously belong to your problem domain, and you should treat them as such and identify them. You should also have functionality allowing to retrieve all cars having certain engine_id and so on. According to how you state the problem, it is ambiguous if all 4 ids define a car.

2) If I understand your question correctly, yes, ids are a critical part of a REST design. This doesn't mean that you can't have functionality to display lists of subordinate objects or even searches.

Answer 2

Make the user first construct a valid Transmission, Engine, Body, and Wheels. Then require them to pass in either IDs or URLs to those resources when constructing the Car.