Advanced EF Core Mapping — Value Objects, Owned Types and Table Splitting

Advanced EF Core mapping unlocks domain-driven design patterns in the data layer. Owned types map value objects (which have no identity of their own) to columns in the owner’s table or a related table. Value converters map C# types that have no direct SQL Server equivalent. Global query filters automatically apply WHERE conditions to all queries for a given entity — the standard mechanism for soft-delete and multi-tenancy.

Advanced Mapping Patterns #

// ── Owned types — value objects in the same table ─────────────────────────
public class Post
{
    public int    Id          { get; set; }
    public string Title       { get; set; } = "";
    public AuthorInfo Author  { get; set; } = new();  // owned type
}

public class AuthorInfo  // no Id — not an entity, a value object
{
    public string DisplayName { get; set; } = "";
    public string? AvatarUrl  { get; set; }
    public string? Bio        { get; set; }
}

// In DbContext:
modelBuilder.Entity<Post>().OwnsOne(p => p.Author, a => {
    a.Property(x => x.DisplayName).HasColumnName("AuthorName").HasMaxLength(100);
    a.Property(x => x.AvatarUrl).HasColumnName("AuthorAvatarUrl");
});
// Columns in Posts table: AuthorName, AuthorAvatarUrl, Bio (not a separate table)

// ── Value converter — map enum to string ──────────────────────────────────
public enum PostStatus { Draft, Review, Published, Archived }

modelBuilder.Entity<Post>()
    .Property(p => p.Status)
    .HasConversion<string>()       // store as "Draft", "Published" etc.
    .HasMaxLength(20);

// Custom converter for a Money value object:
modelBuilder.Entity<Order>()
    .Property(o => o.TotalAmount)
    .HasConversion(
        money => money.Amount,          // C# Money → decimal for SQL
        amount => new Money(amount));   // decimal from SQL → C# Money

// ── Global query filter — automatic soft delete ───────────────────────────
public class AppDbContext : DbContext
{
    protected override void OnModelCreating(ModelBuilder mb)
    {
        // All queries on Posts automatically add WHERE IsDeleted = 0
        mb.Entity<Post>().HasQueryFilter(p => !p.IsDeleted);

        // Access deleted posts when needed (use IgnoreQueryFilters()):
        // var deleted = _db.Posts.IgnoreQueryFilters().Where(p => p.IsDeleted);
    }
}

// ── Shadow properties — audit fields without domain model pollution ────────
modelBuilder.Entity<Post>()
    .Property<DateTime>("CreatedAt")    // not in Post class — shadow property
    .HasDefaultValueSql("SYSUTCDATETIME()");

// Set in SaveChangesInterceptor:
_db.Entry(post).Property("CreatedAt").CurrentValue = DateTime.UtcNow;

// ── Table-per-hierarchy (TPH) inheritance ─────────────────────────────────
// All derived types in one table with a discriminator column
public abstract class Notification { public int Id { get; set; } }
public class EmailNotification : Notification { public string Email { get; set; } = ""; }
public class PushNotification  : Notification { public string DeviceToken { get; set; } = ""; }

modelBuilder.Entity<Notification>()
    .HasDiscriminator<string>("Type")   // Type column: "Email" or "Push"
    .HasValue<EmailNotification>("Email")
    .HasValue<PushNotification>("Push");

Common Mistakes #

Mistake 1 — Enum stored as integer (breaks when enum members are reordered) #

❌ Wrong — Status stored as int; Draft=0, Review=1; team inserts Archive=0 before Draft; all statuses shift.

✅ Correct — .HasConversion<string>(); stored as “Draft”, “Review” — immune to enum reordering.

Mistake 2 — Global query filter applied but raw SQL bypasses it (inconsistent results) #

❌ Wrong — HasQueryFilter for IsDeleted but FromSqlRaw queries return deleted records; inconsistent soft-delete behaviour.

✅ Correct — add filter conditions explicitly to raw SQL, or avoid raw SQL for soft-deleted entity types.