I know this should be easy to find but I can't seem to find it anywhere. I'm simply trying to use a variable in my class as part of the where clause when I'm declaring a one-to-many relationship. I assume there's a way to do this but I can't quite figure out the syntax.

The variable has to be a property of the owning object, and then you use it via the <key> element and property-ref attribute, as appropriate. If it's a constant, you can use the where attribute of the set/list/map element.

If that doesn't help, post some specifics. Table shapes and desired java results will allow me to write the mapping for you.

Here's what I have:
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0">

<class name="Supremacy.DataAccess.DbObjects.Railroads, DataAccess" table="railroad">

<composite-id>
<key-property name="BeginRegion" column="beginregion"/>
<key-property name="EndRegion" column="endregion"/>
<key-property name="GameID" column="gameid"/>
</composite-id>

<bag name="Into" inverse="true" cascade="all" table="railroad">
<key>
<column name="beginregion"/>
<column name="endregion"/>
<column name="gameid"/>
</key>
<one-to-many class="Supremacy.DataAccess.DbObjects.Railroads, DataAccess"/>
</bag>
</class>

</hibernate-mapping>

(yes this is supposed to be a list of objects of the same type as the base object)

I essentially want a list of all railroads into this one region. I want that determined based off of the "EndRegion" property. So I would like my "Into" list to have a list of all of the other records in this table that have their "endregion" equal to this region's "beginregion." I would like to just add "where='endregion=this.BeginRegion'" or something like that in my bag declaration.

I'm pretty sure that your current map won't allow that. You need a key that doesn't include regions. In fact, I recommend having a RailroadId property and column, and don't use any of GameID, EndRegion or BeginRegion in Railroad's key.

Unfortunately for you, NHibernate is based off Hibernate2, so you can't use property-ref in one-to-many mappings (that's only supported in 3.1+). But you can use a unidirectional many-to-one mapping to accomplish precisely the same thing. The ultimate mapping will be something to this effect:I'm not exactly sure what features NHibernate currently supports, and there are several abbreviations available in recent hibernate release that make this mapping easier to read (property-ref is added all over the place, very handy), but this or something like it should do what you need.

Not an ideal solution but it should work. Thanks