ORM Integration Patterns

Overview

Learn patterns for integrating Entity Framework Core with value objects. Covers OwnsOne, OwnsMany, and Value Converter patterns.

---

Learning Objectives

• Map value objects with EF Core’s OwnsOne pattern
• Map collection value objects with the OwnsMany pattern
• Simple conversion with ValueConverter
• Saving and loading value objects to/from database

---

How to Run

cd Docs/tutorials/Functional-ValueObject/04-practical-guide/02-ORM-Integration/OrmIntegration
dotnet run

---

Expected Output

=== ORM Integration Patterns ===

1. OwnsOne Pattern - Composite Value Object Mapping
────────────────────────────────────────
   Saved user: John Doe
   Email: john@example.com
   Address: Seoul Gangnam-gu Teheran-ro 123 (06234)

2. Value Converter Pattern - Single Value Object Conversion
────────────────────────────────────────
   Product code: EL-001234
   Price: 50,000 KRW

3. OwnsMany Pattern - Collection Value Object Mapping
────────────────────────────────────────
   Customer: Jane Smith
   Order items:
      - Product A: 2 x 10,000
      - Product B: 1 x 25,000

---

Core Code Explanation

1. OwnsOne Pattern

modelBuilder.Entity<User>()
    .OwnsOne(u => u.Address, address =>
    {
        address.Property(a => a.City).HasColumnName("City");
        address.Property(a => a.Street).HasColumnName("Street");
        address.Property(a => a.PostalCode).HasColumnName("PostalCode");
    });

Table structure:

Users
├── Id (PK)
├── Name
├── Email       ← OwnsOne (single column)
├── City        ← OwnsOne Address
├── Street      ← OwnsOne Address
└── PostalCode  ← OwnsOne Address

2. Value Converter Pattern

modelBuilder.Entity<Product>()
    .Property(p => p.Code)
    .HasConversion(
        code => code.Value,              // When saving
        value => ProductCode.CreateFromValidated(value)  // When loading
    );

3. OwnsMany Pattern

modelBuilder.Entity<Order>()
    .OwnsMany(o => o.LineItems, lineItem =>
    {
        lineItem.Property(l => l.ProductName);
        lineItem.Property(l => l.Quantity);
        lineItem.Property(l => l.UnitPrice);
    });

Table structure:

Orders                  OrderLineItems
├── Id (PK)            ├── OrderId (FK)
└── CustomerName       ├── ProductName
                       ├── Quantity
                       └── UnitPrice

---

Pattern Selection Guide

Pattern	When to Use	Pros	Cons
OwnsOne	Composite value object	Stored in same table	Column count increases
OwnsMany	Collection value object	Normalized structure	Separate table required
ValueConverter	Single value object	Simple conversion	Cannot handle composite types

FAQ

Q1: Should I use OwnsOne or ValueConverter?

A: ValueConverter is simpler for value objects with a single property (Email, ProductCode, etc.). Use OwnsOne for composite value objects with multiple properties (Address, Money, etc.) to map each property to a separate column.

Q2: Why is CreateFromValidated used when loading with ValueConverter?

A: Values stored in the database have already passed validation, so there is no need to validate again. Using Create would incur unnecessary validation cost and require Fin<T> unwrapping. CreateFromValidated skips validation and creates the instance directly.

Q3: Are there performance issues since OwnsMany creates a separate table?

A: Query performance may be slightly lower than a single table since JOINs are required. However, EF Core automatically manages the relationships, and in practice it rarely becomes a problem if the collection size is not large. For performance-critical queries, it is common to use a separate read model.

---

Next Steps

Learn CQRS integration patterns.

-> 4.3 CQRS Integration