https://github.com/dotnet/roslyn/issues/57852

Warehouse.linq

void Main()
{
    using var context = new WarehouseContext();
    context.Database.EnsureDeleted();
    context.Database.EnsureCreated();
    
    var apple = new Product { Name = "Apple" };
    var pear = new Product { Name = "Pear" };
    context.Products.AddRange(apple, pear);
    
    context.WarehouseStocks.Add(new() { Product = apple, Count = 1 });
    
    context.SaveChanges();
    
    var products = context.Products;
    var warehouseStocks = context.WarehouseStocks;

    var query =
                from product in products
                join warehouseStock in warehouseStocks on product.Id equals warehouseStock.ProductId into ws
                from warehouseStock in ws.DefaultIfEmpty()
                where (warehouseStock.Id == null)
                select product;

    query.ToQueryString().Dump();

    query.Dump();

    var query2 =
                from product in products
                join warehouseStock in warehouseStocks on product.Id equals warehouseStock.ProductId into ws
                from warehouseStock in ws.DefaultIfEmpty()
                where (warehouseStock == null)
                select product;

    query2.ToQueryString().Dump();

    query2.Dump();
}

public class WarehouseContext : DbContext
{
    public DbSet<Product> Products { get; set; }
    public DbSet<WarehouseStock> WarehouseStocks { get; set; }

    public string DbPath { get; }

    public WarehouseContext()
    {
        var path = Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData);
        DbPath = Path.Join(path, "warehouse.db");
    }

    protected override void OnConfiguring(DbContextOptionsBuilder options)
        => options.UseSqlite($"Data Source={DbPath}");
}

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class WarehouseStock
{
    public int Id { get; set; }
    public int ProductId { get; set; }
    public Product Product { get; set; }
    public int Count { get; set; }
}