Why should you care?

ShopNest checkout loads related data; relationships generate FK constraints and enable Include without manual join SQL.

See it live — copy this example

Paste into a .NET project with EF Core packages, then run with LocalDB/SQL Server (dotnet ef / dotnet run).

public class OrderItem
{
    public int Id { get; set; }
    public int OrderId { get; set; }
    public Order Order { get; set; } = null!;
    public int ProductId { get; set; }
    public Product Product { get; set; } = null!;
    public int Quantity { get; set; }
}

What happened?

• OrderId and ProductId are FK columns.
• Order and Product navigations let EF traverse graphs and configure cascades in Fluent API.

Practice next

1. Wire Order ↔ Customer and OrderItem ↔ Product navigations both ways.
2. Configure delete behavior in Fluent API for Order–OrderItem.
3. Add migration and verify FK constraints in SSMS.
4. Make OrderItem–Product relationship optional with nullable ProductId (if business allows).
5. Add shadow property FK via Fluent API without C# property.

Remember

FK property + navigation defines a relationship. Configure delete behavior explicitly for production. Bidirectional navigations clarify the model.

ShopNest order graph

OrderItem rows tie each purchased Product to an Order with enforced FK integrity.

Outcome: Orphan line items cannot exist — refunds always trace to a product.