Why should you care?

ShopNest products carry multiple tags (Sale, Eco, New) and tags apply to many products — classic many-to-many.

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 Product
{
    public int Id { get; set; }
    public ICollection<Tag> Tags { get; set; } = new List<Tag>();
}

public class Tag
{
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public ICollection<Product> Products { get; set; } = new List<Product>();
}

// EF creates ProductTag join table by convention

What happened?

• Bidirectional ICollection navigations without FK on either side signal many-to-many.
• EF creates ProductTag with ProductId and TagId composite key.

Practice next

1. Add Product and Tag with mutual collections.
2. Migrate and inspect ProductTag join table in SSMS.
3. Add tags: product.Tags.Add(tag); SaveChangesAsync.
4. Replace convention with ProductTag entity holding AddedAt date.
5. Query products having tag: Where(p => p.Tags.Any(t => t.Name == "Sale")).

Remember

Skip navigations auto-create join table. Explicit join entity adds columns on link. Both sides need ICollection navigations.

ShopNest promo tags

Marketing tags Sale on hundreds of products via many-to-many without duplicating tag rows.

Outcome: Filter ?tag=Sale uses join table index efficiently.