Why should you care?

ShopNest category tree (Electronics → Laptops → Gaming) uses self-reference instead of separate table per level.

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 Category
{
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public int? ParentCategoryId { get; set; }
    public Category? Parent { get; set; }
    public ICollection<Category> Children { get; set; } = new List<Category>();
}

modelBuilder.Entity<Category>()
    .HasOne(c => c.Parent)
    .WithMany(c => c.Children)
    .HasForeignKey(c => c.ParentCategoryId)
    .OnDelete(DeleteBehavior.Restrict);

What happened?

• ParentCategoryId nullable FK allows root categories.
• HasOne/WithMany on same entity type configures hierarchy.
• Restrict prevents deleting parent with children.

Practice next

1. Seed root and child categories.
2. Query roots: Where(c => c.ParentCategoryId == null).
3. Recursive CTE or client tree build for full hierarchy display.
4. Load three levels with ThenInclude(c => c.Children).ThenInclude(c => c.Children).
5. Add path column materialized for faster breadcrumb queries.

Remember

Same entity type for parent and child. Nullable FK marks root nodes. Restrict delete on hierarchies usually safest.

ShopNest category tree

Mega menu builds from self-referencing Category rows with ParentCategoryId.

Outcome: Unlimited depth aisles without schema change per level.