Question

Given the tables:

TableA
aId
...

TableB
bId
...

TableC
aId
cId // Maps to TableB.bId

In other words TableA and TableB are only related through an entry in TableC.

I want the following classes:

public class ClassA
{
    aId
    ...
}

public class ClassB
{
    bId
    ...
}

public class ClassD : ClassA
{
   ClassB
}

So that ClassD has all of the properties of ClassA with the addition of a single instance of ClassB.

My initial thought was to use:

<?xml version="1.0" encoding="utf-8"?>
<hibernate-mapping auto-import="false" xmlns="urn:nhibernate-mapping-2.2">
  <class name="ClassD" lazy="false" table="TableA" polymorphism="explicit">
    <id name="aId" column="aId" type="Guid">
      <generator class="CustomGuidGenerator" />
    </id>
    ...
    <many-to-one name="ClassB" column="cId" class="ClassB" cascade="all" not-found="ignore" />
  </class>
</hibernate-mapping>

Obviously, in this mapping, there is no cId in TableA nor is there a way in many-to-one to specify a table. So, how do you map a single entity via an intermediate join table?

Answer 1

Not sure what is exactly in your tables (is TableC.aId a reference to TableA.aId ?), but maybe you can go for a table-per-subclass strategy ( see for example http://docs.huihoo.com/hibernate/nhibernate-reference-1.2.0/inheritance.html )

<class name="ClassA" table="TableA">
  <id name="aId" column="aId" type="Guid">
    <generator class="CustomGuidGenerator" />
   </id>        
    <joined-subclass name="ClassD" table="TableC">
        <key column="aId"/>
        <many-to-one name="ClassB" column="cId" class="ClassB" cascade="all" not-found="ignore" />
    </joined-subclass>
</class>

About ClassB, it should have it’s own mapping file, and the system will be able to compute the table name (TableB) for the many-to-one ClassB property.