Eager loading doesn't work with BackgroundService - .net-core

Question

I'm trying to load related entities in a console application that runs a BackgroundService,
but it doesn't load related entities, I have had this problem for hours now and I just noticed it happens only in the BackgroundService, tried the same DbContext in a web application by injecting the DbContext class in the Index page model, without a problem.
Here's the code from the console application:
the Background Service:
public class MyService : BackgroundService
{
private readonly MyDbContext _context;
public MyService(MyDbContext context)
{
_context = context;
}
protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
//Jobs always empty!
var theBatch = _context.Batches.Include(x => x.Jobs).FirstOrDefault();
}
}
the program file:
class Program
{
static async Task Main(string[] args)
{
using IHost host = CreateHostBuilder(args).Build();
await host.RunAsync();
}
static IHostBuilder CreateHostBuilder(string[] args) => Host.CreateDefaultBuilder(args)
.ConfigureServices((context, services) =>
{
services.AddLogging()
.AddDbContext<MyDbContext>(options => options
.UseSqlServer(context.Configuration.GetConnectionString("MyConnection")))
.AddHostedService<MyService>()
.BuildServiceProvider();
});
}
I'm using .NET 5 and EF Core 5.0.12

Answer 1

What you have should work as written. So double-check your configuration. As #Nkosi points out you should use shorter scopes, but putting one scope in your ExecuteAsync isn't enough, as the Task it returns has the same lifetime as the BackgroundService instance.
Anyway here's a complete repro that works fine in .NET 6.
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
class Program
{
static async Task Main(string[] args)
{
using IHost host = CreateHostBuilder(args).Build();
await host.RunAsync();
}
static IHostBuilder CreateHostBuilder(string[] args) => Host.CreateDefaultBuilder(args)
.ConfigureServices((context, services) =>
{
services.AddLogging()
.AddDbContext<MyDbContext>(options => options
.UseSqlServer(context.Configuration.GetConnectionString("MyConnection")))
.AddHostedService<MyService>()
.BuildServiceProvider();
});
}
public class MyDbContext : DbContext
{
public MyDbContext(DbContextOptions<MyDbContext> options) : base(options)
{ }
#pragma warning disable CS8618 // Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
public DbSet<Batch> Batches { get; set; }
public DbSet<Job> Jobs{ get; set; }
#pragma warning restore CS8618 // Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
}
public class Job
{
public int Id { get; set; }
}
public class Batch
{
public int Id { get; set; }
public virtual ICollection<Job> Jobs { get; set; } = new HashSet<Job>();
}
public class MyService : BackgroundService
{
private readonly MyDbContext _context;
public MyService(MyDbContext context)
{
_context = context;
context.Database.EnsureDeleted();
context.Database.EnsureCreated();
var batch = new Batch();
batch.Jobs.Add(new Job() );
context.Batches.Add(batch);
context.SaveChanges();
context.ChangeTracker.Clear();
}
protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
var theBatch = await _context.Batches.Include(x => x.Jobs).FirstOrDefaultAsync();
Console.WriteLine(theBatch.Jobs.Count);
}
}

Answer 2

OK, I'm writing this while I'm really angry for the hours and the effort spent on this problem, I don't know who to blame, Microsoft or the person who installed EF 6 in .NET core project. Microsoft made it difficult to find out the source of the problem.
I was using Include from System.Data.Entity namespace not Microsoft.EntityFrameworkCore
I figured it out by a mere coincidence, I used FirstOrDefaultAsync without a specific reason, and got this weird exception:
'The provider for the source IQueryable doesn't implement
IDbAsyncQueryProvider. Only providers that implement
IDbAsyncQueryProvider can be used for Entity Framework asynchronous
operations.
When I looked at it, I found that I was using the wrong namespace for FirstOrDefaultAsync (System.Data.Entity instead of Microsoft.EntityFrameworkCore)
when I changed the namespace, everything worked!