DDD: aggregate root question

Question

Let's say i got 2 entities - Foo and Bar. Foo is an aggregate root and contains Bar. As far as i understand, it should look like this:

public class Foo{
    private readonly Bar Bar;
}

I want to provide functionality for users to choose Bars for Foos from a defined list (and change it).

If repositories are supposed to be for aggregate roots only it means that there will be no repository for Bar entity.

This leads to problem - Bar can't be created/updated independently without a reference to Foo.

Does that mean that Bar is supposed to have a repository despite that it has no meaning without a Foo?

Answer 1

If you want to select from a list of Bars where they're not associated with Foo, then this is not an aggregate root. For example, you can't get list of OrderItems without their Order, so this is single aggregate root (Order), but you can get list of Products to assign to OrderItems, so Product is not part of the Order aggregate root.

Notice that while OrderItem is part of Order aggregate root, you can still create and update it independently. But, you cannot get it without reference to Order. Same for your Bar, even if it was part of Foo, you could get each(Foo.Bars) and work with it, or do Foo.AddBar(new Bar()). But if you need to get List without Foo, Bar is not part of Foo aggregate. It is a separate entity.

Well, that's how I see DDD here, but I'm not Eric Evans, of course.