ersatztv/ErsatzTV.Application/Watermarks/Commands/DeleteWatermarkHandler.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;
using Unit = LanguageExt.Unit;

namespace ErsatzTV.Application.Watermarks.Commands
{
    public class DeleteWatermarkHandler : MediatR.IRequestHandler<DeleteWatermark, Either<BaseError, Unit>>
    {
        private readonly IDbContextFactory<TvContext> _dbContextFactory;

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

        public async Task<Either<BaseError, Unit>> Handle(
            DeleteWatermark request,
            CancellationToken cancellationToken)
        {
            await using TvContext dbContext = _dbContextFactory.CreateDbContext();
            Validation<BaseError, ChannelWatermark> validation = await WatermarkMustExist(dbContext, request);
            return await validation.Apply(p => DoDeletion(dbContext, p));
        }

        private static async Task<Unit> DoDeletion(TvContext dbContext, ChannelWatermark watermark)
        {
            await dbContext.Database.ExecuteSqlRawAsync(
                $"UPDATE Channel SET WatermarkId = NULL WHERE WatermarkId = {watermark.Id}");
            dbContext.ChannelWatermarks.Remove(watermark);
            await dbContext.SaveChangesAsync();
            return Unit.Default;
        }

        private static Task<Validation<BaseError, ChannelWatermark>> WatermarkMustExist(
            TvContext dbContext,
            DeleteWatermark request) =>
            dbContext.ChannelWatermarks
                .SelectOneAsync(p => p.Id, p => p.Id == request.WatermarkId)
                .Map(o => o.ToValidation<BaseError>($"Watermark {request.WatermarkId} does not exist"));
    }
}