Why should you care?

ShopNest needs unique SKU indexes and precise cascade rules — Fluent API centralizes that configuration away from domain entities.

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).

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Product>(e =>
    {
        e.Property(p => p.Name).HasMaxLength(200).IsRequired();
        e.Property(p => p.Price).HasPrecision(18, 2);
        e.HasIndex(p => p.Sku).IsUnique();
        e.HasOne(p => p.Category)
            .WithMany(c => c.Products)
            .HasForeignKey(p => p.CategoryId)
            .OnDelete(DeleteBehavior.Restrict);
    });
}

What happened?

• OnModelCreating runs once at model build.
• HasPrecision controls decimal columns.
• HasIndex creates a unique index on Sku.
• OnDelete Restrict prevents accidental category deletes.

Practice next

1. Move one annotation from Product to Fluent API.
2. Add unique index on Sku and migrate.
3. Configure Category–Product relationship with explicit delete behavior.
4. Extract Product config to ProductConfiguration : IEntityTypeConfiguration.
5. Add a composite index on CategoryId + Name for catalog search.

Remember

Fluent API lives in OnModelCreating. Best for indexes, relationships, and precision. Keeps domain entities clean.

ShopNest SKU uniqueness

Fluent API adds unique index on Product.Sku so duplicate warehouse codes fail at insert.

Outcome: Database enforces catalog integrity even if API validation is bypassed.