how to map ilist<string> variable which is a member of multiple classes using automapping

suppose i have classes of structure similar to this.

public class baseclass
{

public virtual string Id;
public virtual ilist<string> mStrList;
public baseclass()
{
    mStrList = new list<string>();   
}

}

public class class1 : baseclass
{

}

public class class2 : baseclass
{

}

Saving the objects of type class1 and class2 will FAIL. The reason is when table for mStrList is created the class1_id and class2_id fields are defined as non-nullable but at any time when row is saved only one fk_id can be populated and this cause the save operation to fail. (TESTED in MySql).

I was wondering there was a way to do this is automapping using overrides or something. Currenltly working with hbm.xml mapping files setting property nullable as false