// The Line which thows.
var result = await _context.Todos.Include(x=>x.Category).ToListAsync(); 



public class Category
{
    public ObjectId _id { get; set; }
    public string Name { get; set; }
    public ICollection<Todo> Todos { get; set; }
}

public class Todo
{
    public ObjectId _id { get; set; }
    public string Name { get; set; } = null!;
    public string Description { get; set; } = null!;
    public bool IsDone { get; set; }
    public ObjectId CategoryId { get; set; }
    public Category Category { get; set; }
}

public class TodoDbContext:DbContext
{
    public DbSet<Todo> Todos { get; init; } = null!;
    public DbSet<Category> Categories { get; init; } = null!;
    
    
    public TodoDbContext(DbContextOptions<TodoDbContext> options):base(options)
    {
        
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
        modelBuilder.Entity<Todo>()
            .HasOne(t => t.Category)
            .WithMany(c => c.Todos)
            .HasForeignKey(t => t.CategoryId)
            .IsRequired();

        modelBuilder.Entity<Category>()
            .HasMany(c => c.Todos)
            .WithOne(t => t.Category);

    }
}

builder.Services.AddDbContext<TodoDbContext>(opt =>
{
    var database = new MongoClient(builder.Configuration.GetConnectionString("Default")).GetDatabase("Todos");
    opt.UseMongoDB(database.Client, database.DatabaseNamespace.DatabaseName);
});