I'm not well versed in domain driven design and I've recently started created a domain model for a project. I still haven't decided on an ORM (though I will likely go with NHibernate) and I am currently trying to ensure that my Value Objects should be just that.
I have a few VOs that have almost no behavior other than to encapsulate "like" terms, for instance:
public class Referral {
public Case Case { get; set; } // this is the a reference to the aggregate root
public ReferralType ReferralType { get; set; } // this is an enum
public string ReferralTypeOther { get; set; }
} // etc, etc.
This particular class has a reference to "Case" which is two levels up, so if say I were going to access a Referral I could go: case.social.referral (Case, Social and Referral are all classes, there is a single Social inside a Case and there is a single Referral inside a Social). Now that I am looking at it as I type it, I don't think I need a Case in the Referral since it will be accessible through the Social entity, correct?
Now, there is no doubt in my mind this is something that should be a VO, and the method I plan to use to persist this to the database is to either have NHibernate assign it a surrogate identifier (which I am still not too clear on, if anyone could please elaborate on that too it would help me out, since I don't know if the surrogate identifier requires that I have an Id in my VO already or if it can operate without one) and/or a protected Id property that would not be exposed outside the Referral class (for the sole purpose of persisting to the DB).
Now on to my title question: Should a VO have a collection, (in my case a List) inside it? I can only think of this as a one-to-many relationship in the database but since there is no identity it didn't seem adequate to make the class an entity. Below is the code:
public class LivingSituation {
private IList<AdultAtHome> AdultsAtHome { get; set; }
public ResidingWith CurrentlyResidingWith { get; set } // this is an enum
} // etc, etc.
This class currently doesn't have an Id and the AdultsAtHome class just has intrinsic types (string, int). So I am not sure if this should be an entity or if it can remain as a VO and I just need to configure my ORM to use a 1:m relationship for this using their own tables and a private/protected Id field so that the ORM can persist to the DB.
Also, should I go with normalized tables for each of my classes, or not? I think I would only need to use a table per class when there is a possibility of having multiple instances of the class assigned to an entity or value object and/or there is the possibility of having collections 1:m relationships with some of those objects. I have no problem with using a single table for certain value objects that have intrinsic types but with nested types I think it would be advantageous to use normalized tables. Any suggestions on this as well?
Sorry for being so verbose with the multiple questions:
1) Do I need a surrogate identifier (with say NHibernate) for my value objects?
2) If #1 is yes, then does this need to be private/protected so that my value object "remains" a value object in concept?
3) Can a value object have other value objects (in say, a List) or would that constitute an entity? (I think the answer to this is no, but I'd prefer to be sure before I proceed further.)
4) Do I need a reference to the aggregate root from a value object that is a few levels down from the aggregate root? (I don't think I do, this is likely an oversight on my part when writing the model, anyone agree?)
5) Is it OK to use normalized tables for certain things (like nested types and/or types with collections as properties which would need their own tables anyway for the 1:m relationship) while having the ORM do the mapping for the simpler value objects to the same table that belongs to my entity?
Thanks again.