This may be a silly question, but is there a way of mapping an entity that can be null but that does NOT have a field on the parent table? For example:

parent table
id int(11)

child table
parent_id int(11)
description text

--------------------

class Parent
{
Child child; // Can be null
}

class Child
{
Parent parent; // Cannot be null
string Description;
}

This could be mapped with a set, but I can't figure out whether this kind of linkage is possible for a single element.

Thanks,

Steve

It's still technically a many-to-one relationship. What if there where two entries in the Child table with the same parent_id? Which one would
class Parent
{
Child child;
}

refer too? I don't think you want nHibernate to determine that for you

Hi,

True, but in my (real-world) case the child table has a primary key on parent_id such that it can't have more than one entry per Parent. I wouldn't want to represent that in code with any kind of collection. A one-to-one member doesn't really represent it properly either since it's really one-to-zero.

Steve

You can define this as a one-to-one relationship, but you may need to make the child "constrained" by the parent -- i.e. its ID will always equal the ID of the parent, and NHibernate will set it for you. This kind of mapping is commonly used when the "child" contains a non-required blob.

Unfortunately, there is currently no way in NHibernate 1.x to get it to lazy-load these one-to-one "child" entities. Java Hibernate 3.x supports it, though, and it's a major JIRA item many of us are waiting to see implemented ...

Hi,

Yes, one-to-one is as close as I got, although it doesn't quite model it (as the row should be optional in our model), but thanks.

Steve

How about one-to-many with unique = true?

Unique doesn't seem to be a property for one-to-many in the docs, and in any case that would require modelling the property as a collection. I'll keep having a poke around. I think in my case the <join> tag would achieve my goal, but unfortunately that's not on release yet.