Best way of modelling a list within a component in NHibernate - nhibernate

Question

Imagine I have a property site which lets you enquire to different estate agents about a given property. Different enquiry methods might have different billing calculations associated with them, and not all agents will have every billing model enabled.
public class EmailEnquiryBillingModel : ValueObject
{
public string EmailAddress { get; set; }
public decimal CostPerEnquiry { get; set; }
}
public enum DayOfWeek
{
Monday,
Tuseday,
// etc.
}
public class OpeningHours : ValueObject
{
public DateTime OpeningTime { get; set; }
public DateTime ClosingTime { get; set;}
}
public class PhoneEnquiryBillingModel : ValueObject
{
public PhoneEnquiryBillingModel()
{
OpeningHours = new Dictionary<DayOfWeek, OpeningHours>();
}
public int PhoneNumber { get; set; }
public IDictionary<DayOfWeek, OpeningHours> OpeningHours { get; set; }
}
public class EstateAgent : Entity
{
public string Name { get; set; }
public EmailEnquiryBillingModel EmailEnquiryBillingModel { get; set; }
public PhoneEnquiryBillingModel PhoneEnquiryBillingModel { get; set; }
}
NHibernate has semantics with components (value objects) whereby if every property in a component is null, the component will also be null.
Therefore, with the appropriate mapping, you can write if(estateAgent.EmailEnquiryBillingModel != null) rather than having to check every individual property of the email enquiry billing model, or whether the model is valid: we either have the model or we don't. It's a simple, elegant way of checking if a particular billing model is enabled.
The problem comes when you have a set within a component, such as with the phone enquiry billing model and the various opening hours. Neither the PhoneEnquiryBillingModel nor the OpeningHours are entities. These are legitimate value objects: we don't care whether the estate agent is open from this 9am on Monday or that 9am on a Monday, just that it opens at 9am on a Monday.
Therefore, this feels like the semantically correct way of representing this domain model in C#.
However, the fact that the PhoneEnquiryBillingModel contains a set (of ProviderOpenHours), and a set cannot be null in NHibernate, only empty, means that ProviderOpenHours will always be non-null, even if the estate agent doesn't meaningfully have that enquiry model enabled. (For more info, see: https://ayende.com/blog/4685/those-are-the-rules-even-when-you-dont-like-them).
This means that you can't do a simple check like if(estateAgent.PhoneEnquiryBillingModel != null), because that object is always not-null.
Therefore, for some billing models you'd be able to do a null check to see if they are enabled, but for other billing models you'd have to find an alternative way of checking, depending on whether those billing models contain a set.
Effectively, you'd need to know the internal structure of a billing model to know if you could do that kind of comparison, which feels like you're breaking encapsulation and changing your domain model based on the rules of the ORM.
Is there a better way of modelling this? Or a way to get NHibernate to serialize in the PhoneEnquiryBillingModel as null, if it doesn't have a phone number or any opening hours?

Answer 1

Therefore, with the appropriate mapping, you can write
if(estateAgent.EmailEnquiryBillingModel != null)
This would not be the best encapsulation in itself.
Instead, you go for:
if (estateAgent.DoesAcceptEmailEnquiries())
and
if (estateAgent.DoesAcceptPhoneEnquiries())
This would give better encapsulation than interrogating properties on the EstageAgent aggregate to make an assumption about the EstateAgent's capabilities. What if you decided to change the implementation of how an EstateAgent stored this information internally? You need to change all clients.
There's nothing particularly bad about an EstateAgent performing individual property checks on its underlying value objects.
However, you could go further and implement a checker method on the PhoneEnquiryBillingModel, perhaps even static to avoid the null check in EstateAgent.
PhoneEnquiryBillingModel
public class PhoneEnquiryBillingModel : ValueObject
{
public PhoneEnquiryBillingModel()
{
OpeningHours = new Dictionary<DayOfWeek, OpeningHours>();
}
public int PhoneNumber { get; set; }
public IDictionary<DayOfWeek, OpeningHours> OpeningHours { get; set; }
public static bool DoesAcceptEnquiries(PhoneEnquiryBillingModel phone)
{
if (phone == null) return false;
if (phone.OpeningHours.Count == 0) return false;
return true;
}
}
Estate Agent
public class EstateAgent : Entity
{
public string Name { get; set; }
public EmailEnquiryBillingModel _emailEnquiryBillingModel { get; set; }
public PhoneEnquiryBillingModel _phoneEnquiryBillingModel { get; set; }
public bool DoesAcceptPhoneEnquiries()
{
return PhoneEnquiryBillingModel.DoesAcceptEnquiries(
_phoneEnquiryBillingModel);
}
}