Programming to interfaces while mapping with Fluent NHibernate

Question

I have been whipped into submission and have started learning Fluent NHibernate (no previous NHibernate experience). In my project, I am programming to interfaces to reduce coupling etc. That means pretty much "everything" refers to the interface instead of the concrete type (IMessage instead of Message). The thought behind this is to help make it more testable by being able to mock dependencies.

However, (Fluent) NHibernate doesn't love it when I try to map to interfaces instead of concrete classes. The issue is simple - according to the Fluent Wiki, it is smart to define the ID field of my class as for instance

int Id { get; private set; } 

to get a typical auto-generated primary key. However, that only works with concrete classes - I can't specify an access level on an interface, where the same line has to be

int Id { get; set; } 

and I guess that negates making the setter private in the concrete class (the idea being that only NHibernate should ever set the ID as assigned by the DB).

For now, I guess I will just make the setter public and try to avoid the temptation of writing to it.. But does anyone have an idea of what would be the "proper", best-practice way to create a proper primary-key field that only NHibernate can write to while still only programming to interfaces?

UPDATED

From what I understand after the two answers below from mookid and James Gregory, I may well be on the wrong track - there shouldn't be a reason for me to have an interface per entity as I have now. That's all well and good. I guess my question then becomes - is there no reason to program 100% against an interface for any entities? And if there is even a single situation where this could be justified, is it possible to do this with (Fluent) NHibernate?

I ask because I don't know, not to be critical. Thanks for the responses. :)

Answer 1

I realise this is a diversion, and not an answer to your question (although I think mookid has got that covered).

You should really evaluate whether interfaces on your domain entities are actually providing anything of worth; it's rare to find a situation where you actually need to do this.

For example: How is relying on IMessage any less coupled than relying on Message, when they both (almost) undoubtedly share identical signatures? You shouldn't need to mock an entity, because it's rare that it has enough behavior to require being mocked.