using Microsoft.EntityFrameworkCore;
public sealed class LabDb : DbContext
{
    public string TenantId { get; }
    public LabDb(DbContextOptions<LabDb> options, string tenantId) : base(options) { if (string.IsNullOrWhiteSpace(tenantId)) throw new System.ArgumentException("Trusted tenant required"); TenantId = tenantId; }
    public DbSet<Item> Items => Set<Item>(); public DbSet<Reservation> Reservations => Set<Reservation>();
    protected override void OnModelCreating(ModelBuilder model)
    {
        model.Entity<Item>().HasIndex(x => new { x.TenantId, x.Sku }).IsUnique();
        model.Entity<Item>().Property(x => x.Version).IsConcurrencyToken();
        model.Entity<Item>().HasQueryFilter("Tenant", x => x.TenantId == TenantId);
        model.Entity<Reservation>().HasQueryFilter("Tenant", x => x.TenantId == TenantId);
        model.Entity<Reservation>().HasIndex(x => new { x.TenantId, x.Reference }).IsUnique();
        model.Entity<Reservation>().HasOne(x => x.Item).WithMany().HasForeignKey(x => x.ItemId).OnDelete(DeleteBehavior.Restrict);
    }
}
public sealed class Item { public int Id { get; set; } public string TenantId { get; set; } = ""; public string Sku { get; set; } = ""; public int Stock { get; set; } public int PriceCents { get; set; } public int Version { get; set; } }
public sealed class Reservation { public int Id { get; set; } public string TenantId { get; set; } = ""; public string Reference { get; set; } = ""; public int ItemId { get; set; } public Item Item { get; set; } = null!; public int Quantity { get; set; } public long TotalCents { get; set; } }