DDD, value objects and ORM

Question

Value objects do not have identity. ORM needs identity to update the database.

How to trick ORM?

(Marking Id for value object as internal won't work because ORM lives in a different assembly and moving it to the same assembly is not acceptable).

Thanks in advance.

Answer 1

As far as my understanding of DDD goes value objects are just a way to partition your entities. If a value object should be stored with an ID in the database it's not a value object.

Example:

The domain model looks like this (C#):

public class Customer : Entity
{
    public Guid CustomerID { get; }

    public string LastName { get; set; }

    public Address HomeAddress { get; set; }
}

public class Address : ValueObject
{
    public string Street { get; set; }

    public string City { get; set; }

    public string ZipCode { get; set; }
}

The corresponding database table would look something like this (Pseudo-SQL):

CREATE TABLE Customers
(
    CustomerID,

    LastName,

    HomeAddress_Street,

    HomeAddress_City,

    HomeAddress_ZipCode,
)

To store the addresses in a seperate table you would make it an entity which has an ID.

Answer 2

When Eric Evans talks about "entities have identity, Value Objects do not", he's not talking about an ID column in the database - he's talking about identity as a concept.

VOs have no conceptual identity. That doesn't mean that they shouldn't have persistence identity. Don't let persistence implementation cloud your understanding of Entities vs VOs.

See my post here.

Answer 3

Personally I have the Id field in the value object - I treat it as another attribute of the value object (such as name, location etc).

It may not be true DDD but it works for me.