Wednesday, January 21, 2015

NHibernate: Associations with Composite Primary Keys as Part of a Composite Primary Key

NHibernate is a pretty useful tool, but occasionally it's not entirely documented in a way that makes it's flexibility evident. Composite keys are a particularly difficult area in this regard, as evidenced by the numerous articles on the topic.

Most of the existing articles cover this simply enough, but there is one uncommon corner case I have yet to see explained anywhere: a composite primary key one of whose key properties is an association with a composite key. This is probably pretty uncommon and there are ways around it, hence the lack of examples, but as a testament to NHibernate's flexibility, it's possible! Here's the example in code listing only the primary keys:

public class MotorType
{
  public Horsepower Horsepower { get; protected set; }
  public VoltageType VoltageType { get; protected set; }
}
public class Motor
{
  public MotorType MotorType { get; protected set; }
  public Efficiency Efficiency { get; protected set; }
}

The tables look like this:

MotorType
HorsepowerIdVoltageTypeId
Motor
HorsepowerIdVoltageTypeIdEfficiencyId

Most people would probably map this with the Motor class having the three primary key properties, one for each column, and an additional many-to-one association referencing MotorType, but that shouldn't actually be necessary. Composite primary keys are possible, and the primary key can contain an association, therefore it should be possible, in principle, for that association to itself need a composite key.

And here's how it's done for the Motors table:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
  <class name="Motor, ExampleProject" table="Motors">
    <composite-id>
      <key-many-to-one name="MotorType">
        <column name="HorsepowerId" />
        <column name="VoltageTypeId" />
      </key-many-to-one>
      <key-property name="Efficiency" column="EfficiencyId" />
    </composite-id>
  </class>
</hibernate-mapping>

The MotorType class is a simple composite primary key:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
<class name="MotorType, ExampleProject" table="MotorTypes">
    <composite-id>
      <key-property name="Horsepower" column="HorsepowerId" />
      <key-property name="VoltageType" column="VoltageTypeId" />
    </composite-id>
</class>
</hibernate-mapping>