NHibernate constraints associations of a parent class are dropped on the subclass - nhibernate

Question

I think that the main issue is that the subclass deletes all constraints related to the base mapped collections.
Using the table per concrete class strategy, I have found that the parent collections are not asociated with the subclasses also in another (maybe related) problem, the associations between Basetypes and ChildTypes are not created either.
I have a schema similar to this:
public class Parent{
public virtual Int64 Id{get; set;}
public virtual IList<Foo> foos{get; set;}
public virtual IList<ParentType> _pts{get; set;}
}
public class child: Parent{
public virtual int chilInt{get; set;}
}
public class BaseType{
public virtual Int64 Id{get; set;}
public virtual Parent ParentReference{get; set;}
}
public class ChildType: BaseType{
public virtual string childBacon{get; set;}
}
Mapping Files
<class name="Parent" abstract="true">
<id name="Id" type="Int64" column="Id" unsaved-value="0">
<generator class="native"/>
</id>
<set name="foos" inverse="false" >
<key column="Id"/>
<one-to-many class="Foo" />
</set>
<set name="pts" inverse="false" >
<key column="Id"/>
<one-to-many class="ParentType" />
</set>
</class>
<union-subclass name="Child" table="Child" extends="Parent">
<property name="childInt" type="int" />
</union-subclass>
<class name="ParentType" abstract="true">
<id name="Id" type="Int64" column="Id" unsaved-value="0">
<generator class="native"/>
</id>
<many-to-one name="ParentReference" class="Parent"/>
</class>
<union-subclass name="ChildType" table="ChildType" extends="ParentType">
<property name="childBacon" type="string" />
</union-subclass>
The result that the child table don't have any relation with foo table.

Answer 1

If you use the <union-subclass> mapping it's clear that there is no direct relation of the foo entries to your child table because the child table only contains the additional things declared in the child class.
When instantiating a child instance with union-subclass mapping you get a row in both, the parent and child tables. And if your child instance contains entries in the foo set, you get some rows in the foo table with relation to the parent table.
Using table per concrete class mapping does not make sense with associations pointing to the parent class (as the foo class not part of your code example seems to do) because then the different derived classes of parent all inherit the foo set but the foo table cannot have foreign keys to all those tables.

Answer 2

Well, there are three common approaches for ORM and inheritance (table per class hierarchy, table per subclass, table per concrete class). <union-subclass>, one you use, is used in table per concrete class and it should be embedded in parent <class>. Read about it here (8.1.5).
Maybe it won't resolve all your issues, but at least it should help with establishing mapping for inheritance.