NHibernate mapping with two special cases

I am using NHibernate to map a class to a database table. The Part table has an ID column (primary key) and a ParentPart column (along with a few others).

class Part
{
    public virtual long ID{ get; set; }
    public virtual Part ParentPart { get; set; }   
}

The ParentPart is normally another valid part in the part ta开发者_开发知识库ble but I have two special cases. I have a case where the ParentPart column can be 0 (zero) and another case where it can be -1. Neither of these cases currently represent another valid Part object. I was thinking I could make 2 subclasses of Part (ZeroPart and NegativeOnePart) that would never persist. I want the zero and -1 values to be entered in the column but not persist the entire ZeroPart or NegativeOnePart objects. I am unsure how to map this (I'm using hbm files) or if this even the correct approach.

How can I map this so that normal valid parts are persisted but I can also handle the special cases?

As an aside: My current hbm file has the Part.ID's unsaved value as zero but I think I can just change this in the mapping to something different and default it in the class.

If I understand you correctly, you have a tree structure and you want to assign dummy-objects (with ID = 0 or ID = -1) that shall not be persisted to the database when persisting the objects. The ParentPart column shall contain ID = 0 or -1, depending on some criteria.

Based on this information I assume that you do not have foreign-keys in your table, so that saving an object with a ParentPart = 0 or -1 will not result in a violation of referential integrity. Under normal circumstances I would suggest using a foreign key and allowing NULL in the ParentPart column.

However, since you want two different values (0 and -1) as an indicator that no parent exists, I believe that your best option is probably just creating the two entries in the database (both with ParentPart = NULL).

If you strictly do not want to add those entries, you can have a look at intereceptors and events.

I don't think you can solve this within your mapping files. The NHibernate mapping feature is not designed to handle such specific logic, nor should it be able to do so.

(Having that kind of logic in the database may not be very desirable, but it sounds like this is not a new project, so we don't need to argue about it here.)

Will there be Part values in your "parts" table with the specified IDs, to maintain referential integrity?

If so, then I would suggest creating two special Part instances, each representing the special cases your model implies. You would need to fetch these instances from the database when you want to assign them, but this can be made simple with some helper methods on your repositories.