entity-configuration

name	entity-configuration
description	Canonical EF Core Fluent API entity configuration rules. Load when creating or modifying IEntityTypeConfiguration classes.
requires	["backend-dotnet-csharp","dotnet-ddd"]
produces	["entity-configurations","relationship-mappings"]

Entity Configuration — TemperAI

🚨 NON-NEGOTIABLE RULES — ZERO TOLERANCE

1. NEVER use DataAnnotations on entities
2. NEVER use nvarchar(max) or varchar(max) — always use explicit lengths from Entity.Rules
3. NEVER call builder.ToTable() unless a future skill explicitly requires it
4. NEVER use HasDefaultValueSql() or ValueGeneratedOnAdd() for primary keys
5. NEVER use OwnsOne in this taxonomy

Load when

• Creating IEntityTypeConfiguration<T> classes
• Mapping relationships, indexes, precision, or column lengths
• Registering configurations in DbContext

Basic entity configuration

public sealed class ProductConfiguration : IEntityTypeConfiguration<Product>
{
    public void Configure(EntityTypeBuilder<Product> builder)
    {
        builder.HasKey(product => product.Id);

        builder.Property(product => product.Name)
            .HasMaxLength(Product.Rules.NAME_MAX_LENGTH)
            .HasColumnType("varchar");

        builder.Property(product => product.Description)
            .HasMaxLength(Product.Rules.DESCRIPTION_MAX_LENGTH)
            .HasColumnType("nvarchar");

        builder.Property(product => product.Price)
            .HasColumnType("decimal(18,2)");

        builder.Property(product => product.Status)
            .HasConversion<string>()
            .HasMaxLength(20)
            .HasColumnType("varchar");

        builder.Property(product => product.CreatedAt)
            .HasColumnType("datetime2");

        builder.Property(product => product.UpdatedAt)
            .HasColumnType("datetime2");

        builder.HasIndex(product => product.Name)
            .IsUnique();
    }
}

Aggregate with child entities

public sealed class OrderConfiguration : IEntityTypeConfiguration<Order>
{
    public void Configure(EntityTypeBuilder<Order> builder)
    {
        builder.HasKey(order => order.Id);

        builder.Property(order => order.CustomerId)
            .HasColumnType("uniqueidentifier");

        builder.Property(order => order.Status)
            .HasConversion<string>()
            .HasMaxLength(20)
            .HasColumnType("varchar");

        builder.HasMany(order => order.Items)
            .WithOne()
            .HasForeignKey(item => item.OrderId)
            .OnDelete(DeleteBehavior.Cascade);
    }
}

Column type reference

CLR Type	SQL Type	Notes
string ASCII	varchar(n)	Codes, names, identifiers
string Unicode	nvarchar(n)	Descriptions and free text
decimal	decimal(18,2)	Monetary values
DateTime	datetime2	Always UTC
Guid	uniqueidentifier	PKs and FKs
int	int	Counts and quantities
bool	bit	Flags
enum	varchar(n)	Use .HasConversion<string>()

Anti-patterns — never do this

// ❌ NEVER use DataAnnotations
public class Product
{
    [Key]
    [Required]
    [MaxLength(100)]
    public Guid Id { get; set; }
}

// ❌ NEVER use nvarchar(max)
builder.Property(product => product.Description)
    .HasColumnType("nvarchar(max)");

// ❌ NEVER use ToTable here
builder.ToTable("Products");

// ❌ NEVER use default SQL generation for PKs here
builder.Property(product => product.Id)
    .HasDefaultValueSql("NEWID()");