Support POST requests to a to-many relationship link

[avrittrohwer]

As per the json:api spec (under the section linked at http://jsonapi.org/format/#crud-updating-relationships)

A server MUST respond to PATCH, POST, and DELETE requests to a URL from a to-many relationship link
...
If a client makes a POST request to a URL from a relationship link, the server MUST add the specified
members to the relationship unless they are already present. If a given type and id is already in the
relationship, the server MUST NOT add it again

[jaredcnance]

Steps to Implementation

@maurei

For the most part, you can follow the PatchRelationshipsAsync path.

• Define a new service layer interface ICreateRelationshipService (or better name). See the existing interfaces. This interface should also b implemented by IResourceCmdService.

• Add dependency on the new interface to constructor of BaseJsonApiController:

JsonApiDotNetCore/src/JsonApiDotNetCore/Controllers/JsonApiController.cs

Lines 26 to 36 in 6b9d14c

	public JsonApiController(
	IJsonApiContext jsonApiContext,
	IGetAllService<T, int> getAll = null,
	IGetByIdService<T, int> getById = null,
	IGetRelationshipService<T, int> getRelationship = null,
	IGetRelationshipsService<T, int> getRelationships = null,
	ICreateService<T, int> create = null,
	IUpdateService<T, int> update = null,
	IUpdateRelationshipService<T, int> updateRelationships = null,
	IDeleteService<T, int> delete = null
	) : base(jsonApiContext, getAll, getById, getRelationship, getRelationships, create, update, updateRelationships, delete) { }

• Add PostRelationshipAsync to BaseJsonApiController that checks to see if the service is available and if so, forwards the request to the service layer and returns an HTTP 200 status code if successful.

• Add PostRelationshipAsync method to JsonApiController similar to PatchRelationshipsAsync. This method should just be a stub to define the routing and then forward the call on to the base class.

JsonApiDotNetCore/src/JsonApiDotNetCore/Controllers/JsonApiController.cs

Lines 89 to 92 in 6b9d14c

	[HttpPatch("{id}/relationships/{relationshipName}")]
	public override async Task<IActionResult> PatchRelationshipsAsync(
	TId id, string relationshipName, [FromBody] List<ResourceObject> relationships)
	=> await base.PatchRelationshipsAsync(id, relationshipName, relationships);

• Define the implementation of the method in the EntityResourceService. This should be pretty straightforward. We should lookup the resource and relationship on the ResourceGraph and ensure that it is a HasMany relationship. If not, I think we should return an HTTP 403 with a meaningful message.

• The code that actually updates the resource in the database should be defined as a new method in the DefaultEntityRepository. The method should also be defined on the IEntityWriteRepository.

• Add the new interface to auto discovery

JsonApiDotNetCore/src/JsonApiDotNetCore/Graph/ServiceDiscoveryFacade.cs

Line 18 in 6b9d14c

internal static HashSet<Type> ServiceInterfaces = new HashSet<Type> {

• Add the new interface to service registration

JsonApiDotNetCore/src/JsonApiDotNetCore/Extensions/IServiceCollectionExtensions.cs

Lines 115 to 137 in 6b9d14c

	services.AddScoped(typeof(IEntityRepository<>), typeof(DefaultEntityRepository<>));
	services.AddScoped(typeof(IEntityRepository<,>), typeof(DefaultEntityRepository<,>));
	services.AddScoped(typeof(ICreateService<>), typeof(EntityResourceService<>));
	services.AddScoped(typeof(ICreateService<,>), typeof(EntityResourceService<,>));
	services.AddScoped(typeof(IGetAllService<>), typeof(EntityResourceService<>));
	services.AddScoped(typeof(IGetAllService<,>), typeof(EntityResourceService<,>));
	services.AddScoped(typeof(IGetByIdService<>), typeof(EntityResourceService<>));
	services.AddScoped(typeof(IGetByIdService<,>), typeof(EntityResourceService<,>));
	services.AddScoped(typeof(IGetRelationshipService<,>), typeof(EntityResourceService<>));
	services.AddScoped(typeof(IGetRelationshipService<,>), typeof(EntityResourceService<,>));
	services.AddScoped(typeof(IUpdateService<>), typeof(EntityResourceService<>));
	services.AddScoped(typeof(IUpdateService<,>), typeof(EntityResourceService<,>));
	services.AddScoped(typeof(IDeleteService<>), typeof(EntityResourceService<>));
	services.AddScoped(typeof(IDeleteService<,>), typeof(EntityResourceService<,>));
	services.AddScoped(typeof(IResourceService<>), typeof(EntityResourceService<>));
	services.AddScoped(typeof(IResourceService<,>), typeof(EntityResourceService<,>));