What is Owned Entity? When and why to use Owned Entity in Entity Framework Core?

Question

I'm learning Entity Framework Core. I came across the term "Owned Entity" in almost all tutorials.

Here is one example on using an Owned Entity in Entity Framework Core

Job Entity:

public class Job : Entity
{
    public HiringManagerName HiringManagerName { get; private set; }
}

HiringManagerName Value Object:

public class HiringManagerName : ValueObject
{
    public string First { get; }
    public string Last { get; }

    protected HiringManagerName()
    {
    }

    private HiringManagerName(string first, string last)
        : this()
    {
        First = first;
        Last = last;
    }

    public static Result<HiringManagerName> Create(string firstName, string lastName)
    {
        if (string.IsNullOrWhiteSpace(firstName))
            return Result.Failure<HiringManagerName>("First name should not be empty");
        if (string.IsNullOrWhiteSpace(lastName))
            return Result.Failure<HiringManagerName>("Last name should not be empty");

        firstName = firstName.Trim();
        lastName = lastName.Trim();

        if (firstName.Length > 200)
            return Result.Failure<HiringManagerName>("First name is too long");
        if (lastName.Length > 200)
            return Result.Failure<HiringManagerName>("Last name is too long");

        return Result.Success(new HiringManagerName(firstName, lastName));
    }

    protected override IEnumerable<object> GetEqualityComponents()
    {
        yield return First;
        yield return Last;
    }
}

Entity Configuration:

public class JobConfiguration : IEntityTypeConfiguration<Job>
{
    public void Configure(EntityTypeBuilder<Job> builder)
    {
        builder.OwnsOne(p => p.HiringManagerName, p =>
        {
            p.Property(pp => pp.First)
                .IsRequired()
                .HasColumnName("HiringManagerFirstName")
                .HasMaxLength(200);
            p.Property(pp => pp.Last)
                .IsRequired()
                .HasColumnName("HiringManagerLastName")
                .HasMaxLength(200);
        });
    }
}

And this gets created as two columns in table like other columns in Job Entity.

Since this is also created as columns just like other properties in entity this can directly be added as normal properties in the Job Entity. Why this needs to be added as Owned Entity?

Please can anyone help me understand,

1. What is owned entity?
2. Why we need to use owned entity?
3. When to use owned entity?

Answer 1

While @Whit Waldo explanation is great with respect to technical ef core, we should also try to understand from Domain Driven Design perspective.

Lets observe the classes mentioned in the question itself

public class Job : Entity

and

public class HiringManagerName : ValueObject

Take a note at Entity and ValueObject. Both of them are DDD concepts.

Identity matters for entities, but does not matter for value objects.

Take a look at this write up from Vladimir Khorikov for a more extensive explanation.

I past the summary bullets here.

• Entities have their own intrinsic identity, value objects don’t.

• The notion of identity equality refers to entities; the notion of structural equality refers to value objects; the notion of reference equality refers to both.

• Entities have a history; value objects have a zero lifespan.

• A value object should always belong to one or several entities, it can’t live by its own.

• Value objects should be immutable; entities are almost always mutable.

• To recognize a value object in your domain model, mentally replace it with an integer.

• Value objects shouldn’t have their own tables in the database.

• Always prefer value objects over entities in your domain model.

So a value object is owned by an entity. So how do we achieve that using EF Core? Here comes the concept of Owned entities. Now go back and read @Whit Waldo answer.