FanOut sample for Dates

FanOutSample.cs

using Marten;
using Marten.Events.Projections;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

var builder = Host.CreateApplicationBuilder();
builder.Services.AddMarten(opt => {
    opt.Connection(builder.Configuration.GetConnectionString("Marten") ?? "Host=localhost; Port=5433; User Id=postgres; Password=pgsql");
    opt.Projections.Add<WorkByDayProjection>(ProjectionLifecycle.Inline);
}).UseLightweightSessions();

builder.Services.AddHostedService<Runner>();

var app = builder.Build();

await app.RunAsync();

public class Runner : BackgroundService
{
    private readonly IDocumentSession _session;
    private readonly IHostApplicationLifetime _lifetime;

    public Runner(IDocumentSession session, IHostApplicationLifetime lifetime)
    {
        _session = session;
        _lifetime = lifetime;
    }

    protected override Task ExecuteAsync(CancellationToken stoppingToken)
        => Task.Run(async () =>
        {

            var assignmentId = Guid.NewGuid();
            var workerId = Guid.NewGuid();

            var today = DateOnly.FromDateTime(DateTime.Today);
            var tomorrow = today.AddDays(1);
            var oneWeekFromToday = today.AddDays(7);
            
            Console.Write($"{today} {oneWeekFromToday}");

            _session.Events.Append(assignmentId, new WorkerAssigned(assignmentId, workerId, today, oneWeekFromToday));
            _session.Events.Append(assignmentId, new WorkCompleted(assignmentId, today, "Work completed today"));
            _session.Events.Append(assignmentId, new WorkCompleted(assignmentId, today, "More work completed today"));
            _session.Events.Append(assignmentId, new WorkCompleted(assignmentId, tomorrow, "Work completed tomorrow"));

            await _session.SaveChangesAsync(stoppingToken);

            _lifetime.StopApplication();
        }, stoppingToken);
}


public record WorkerAssigned(Guid AssignmentId, Guid WorkerId, DateOnly Start, DateOnly End);

public record WorkCompleted(Guid AssignmentId, DateOnly Date, string DescriptionOfWork);


public record WorkByDay(string Id, Guid AssignmentId, Guid WorkerId, DateOnly Date, string[] WorkCompleted);

public record WorkerAssignedForDay(Guid AssignmentId, Guid WorkerId, DateOnly Date);

public class WorkByDayProjection : MultiStreamProjection<WorkByDay, string>
{
    public WorkByDayProjection()
    {
        FanOut<WorkerAssigned, WorkerAssignedForDay>(e => Enumerable.Range(0, e.End.DayNumber - e.Start.DayNumber + 1)
            .Select(d => new WorkerAssignedForDay(e.AssignmentId, e.WorkerId, e.Start.AddDays(d))), FanoutMode.BeforeGrouping);
        Identity<WorkerAssignedForDay>(e => IdForEvent(e.AssignmentId, e.Date));
        Identity<WorkCompleted>(e => IdForEvent(e.AssignmentId, e.Date));
        IncludeType<WorkerAssigned>();
    }

    private static string IdForEvent(Guid assignmentId, DateOnly date) => $"{assignmentId:N}:{date:yyyyMMdd}";

    public static WorkByDay Create(WorkerAssignedForDay evt) => new(IdForEvent(evt.AssignmentId, evt.Date),
        evt.AssignmentId, evt.WorkerId, evt.Date, Array.Empty<string>());

    public static WorkByDay Apply(WorkCompleted evt, WorkByDay view) =>
        view with { WorkCompleted = view.WorkCompleted.Append(evt.DescriptionOfWork).ToArray() };
}