ersatztv/ErsatzTV.Application/MediaCollections/Commands/DeleteCollectionHandler.cs

using System.Threading;
using System.Threading.Tasks;
using ErsatzTV.Core;
using ErsatzTV.Core.Domain;
using ErsatzTV.Infrastructure.Data;
using ErsatzTV.Infrastructure.Extensions;
using LanguageExt;
using Microsoft.EntityFrameworkCore;

namespace ErsatzTV.Application.MediaCollections.Commands
{
    public class DeleteCollectionHandler : MediatR.IRequestHandler<DeleteCollection, Either<BaseError, Unit>>
    {
        private readonly IDbContextFactory<TvContext> _dbContextFactory;

        public DeleteCollectionHandler(IDbContextFactory<TvContext> dbContextFactory) =>
            _dbContextFactory = dbContextFactory;

        public async Task<Either<BaseError, Unit>> Handle(
            DeleteCollection request,
            CancellationToken cancellationToken)
        {
            await using TvContext dbContext = _dbContextFactory.CreateDbContext();

            Validation<BaseError, Collection> validation = await CollectionMustExist(dbContext, request);
            return await validation.Apply(c => DoDeletion(dbContext, c));
        }

        private static Task<Unit> DoDeletion(TvContext dbContext, Collection collection)
        {
            dbContext.Collections.Remove(collection);
            return dbContext.SaveChangesAsync().ToUnit();
        }

        private static Task<Validation<BaseError, Collection>> CollectionMustExist(
            TvContext dbContext,
            DeleteCollection request) =>
            dbContext.Collections
                .SelectOneAsync(c => c.Id, c => c.Id == request.CollectionId)
                .Map(o => o.ToValidation<BaseError>($"Collection {request.CollectionId} does not exist."));
    }
}