How to support creating links for sub-collections in the PagedListRepresentation sample?

[jasonmitchell]

The Hallo.Sample project includes an example PagedListRepresentation<T> which accepts the baseUrl for the collection. Below is the snippet of how this is constructed.

hallo/Hallo.Sample/Models/Hypermedia/PersonRepresentation.cs

Lines 24 to 28 in db5d14e

	public class PersonListRepresentation : PagedListRepresentation<Person>
	{
	public PersonListRepresentation(PersonRepresentation personRepresentation)
	: base("/people", personRepresentation) { }
	}

This works fine for constant base URLs like in this example but is more difficult when the base URL is dynamic. For example, if the URL to the collection was /race/123/cars (where 123 is the resource id) how do we support getting the race id into the base URL?

[jasonmitchell]

This is pretty fiddly. I think it's going to be tricky to come up with a universal solution for this which remains inline with the intention that Hallo requires no modification to controllers or models, and which doesn't have an over-complicated setup.

Two very similar options are below.

The first requires IActionContextAccessor as a parameter and then accesses the RouteData to find the raceId. This is highly MVC-specific as this interface comes from the Microsoft.AspNetCore.Mvc namespace.

public class CarListRepresentation : PagedListRepresentation<Car>
{
    public CarListRepresentation(CarRepresentation carRepresentation, IActionContextAccessor actionContextAccessor)
        : base($"/races/{actionContextAccessor.ActionContext.RouteData.Values["raceId"]}/cars", carRepresentation)
    { }
}

The second option is basically the same but makes use of Microsoft.AspNetCore.Http.IHttpContextAccessor and so is not specific to MVC though I haven't tested this outside of MVC yet.

public class CarListRepresentation : PagedListRepresentation<Car>
{
    public CarListRepresentation(CarRepresentation carRepresentation, IHttpContextAccessor httpContextAccessor)
        : base($"/races/{httpContextAccessor.HttpContext!.GetRouteValue("raceId")}/cars", carRepresentation)
    { }
}

Both options require intimate knowledge of the route values from the endpoint which could potentially be a fragile dependency - someone might do a seemingly harmless rename on the route value which would quietly break this. It's also pretty ugly looking but it could be tidied up with a developers own abstraction if needs be.

This also increases the risk of weird lifestyle issues if using the ASP.NET Core dependency injection mechanism and a developer registers Hal<T> services as singletons because the base URL in this example would always have the first raceId handled by the endpoint. This could be avoided by a different implementation of PagedListRepresentation<T> which resolves the baseUrl differently (e.g. by an abstract method).

Lastly these options really limit the portability of representations. If we wanted to reuse this same representation for a sub-collection under another resource then we couldn't use the same CarListRepresentation across all (and perhaps this is ok).

I will keep experimenting with options for this though I think some trade offs will have to be accepted in this scenario.

[jasonmitchell]

Example resolving baseUrl via abstract method:

CarListRepresentation:

public class CarListRepresentation : PagedListRepresentation<Car>
{
    private readonly IHttpContextAccessor _httpContextAccessor;

    public CarListRepresentation(CarRepresentation carRepresentation, IHttpContextAccessor httpContextAccessor)
        : base(carRepresentation)
    {
        _httpContextAccessor = httpContextAccessor;
    }

    protected override string GetBaseUrl() =>
        $"/races/{_httpContextAccessor.HttpContext!.GetRouteValue("raceId")}/cars";
}

[jchannon]

That or just use the full url https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.http.extensions.urihelper.getencodedurl

[jchannon]

Actually you won't want that as you'll want relative paths but you could just use the request path?

[jasonmitchell]

Yeah you could. I think there are lots of options for this and I'm not sure that the library can cover all scenarios for everyone. What might help is a better sample which shows a more complex scenario which can at least provide a starting point for people to adapt.

[jchannon]

Yarp!

…

On Thu, 25 Feb 2021 at 19:00, Jason Mitchell ***@***.***> wrote: Yeah you could. I think there are lots of options for this and I'm not sure that the library can cover all scenarios for everyone. What might help is a better sample which shows a more complex scenario which can at least provide a starting point for people to adapt. — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#42 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAAZVJS6PDXRC5BHNNEYYCTTA2M5LANCNFSM4YCZTJXA> .