How to deal with value objects in Entity Framework?

Question

How do I persist value objects in Entity Framework without polluting my domain model? EF (well, relational DBs in general) require me to define a key - which my value objects don't have out of the box, for example

public class Tag : ValueObject<Tag>
{
   private readonly string name;

   public Tag(string name)
   {
      this.name = name;
   }

   public string Name { get { return this.name; }}
}

On the other hand, I shouldn't address persistence concerns in the model. Am I really supposed to create another class that includes all the fields from the value object plus a key property and then map them to each other? I'd rather not.

Is there maybe a more elegant solution?

Answer 1

Vaughn Vernon writes about Persisting Value Objects (page 248) in his excellent book Implementing Domain-Driven Design.

ORM and Single Value Objects

The basic idea is to store each of the attributes of the Value in separate columns of the row where its parent Entity is stored. Said another way, a single Value Object is denormalized into its parent Entity's row. There are advantages to employing convention for column naming to clearly identity and standardize the way serialized objects are named.

ORM and Many Values Backed by a Database Entity

A very straightforward approach to persisting a collection of Value instances using an ORM and a relational database is to treat the Value type as an entity in the data model. (...) To accomplish this we can employ a Layer Supertype.

Sample bounded contexts in C# can be found here: https://github.com/VaughnVernon/IDDD_Samples_NET