I have a rather large inheritance hierarchy in which some of
the subclasses add very
little and others add quite a bit. I don't want to map
the entire hierarchy using either "table per class hierarchy" or "table per subclass" due to
the size and complexity of
the hierarchy. Ideally I'd like to mix mapping strategies such that portions of
the hierarchy where
the subclasses add very
little are combined into a common table a la "table per class hierarchy" and subclasses that add a lot are broken out into a separate table. Using this approach, I would expect to have 2 or 3 tables with very
little wasted space instead of either 1 table with lots of fields that don't apply to most of
the objects, or 20+ tables, several of which would have only a couple of columns.
In
the NHibernate Reference Documentation version 2.1.0, I found section 8.1.4 "Mixing table per class hierarchy with table per subclass". This approach switches strategies partway down
the hierarchy by using:
...
<subclass ...>
<join ...>
<property ...>
...
</join>
</subclass>
...
This is great in theory. In practice, though, I found that
the schema was too restrictive in what was allowed inside
the "join" element for me to be able to accomplish what I needed.
Here is
the related part of
the schema definition:
<xs:element name="join">
<xs:complexType>
<xs:sequence>
<xs:element ref="subselect" minOccurs="0" />
<xs:element ref="comment" minOccurs="0" />
<xs:element ref="key" />
<xs:choice minOccurs="0" maxOccurs="unbounded">
<xs:element ref="property" />
<xs:element ref="many-to-one" />
<xs:element ref="component" />
<xs:element ref="dynamic-component" />
<xs:element ref="any" />
<xs:element ref="map" />
<xs:element ref="set" />
<xs:element ref="list" />
<xs:element ref="bag" />
<xs:element ref="idbag" />
<xs:element ref="array" />
<xs:element ref="primitive-array" />
</xs:choice>
<xs:element ref="sql-insert" minOccurs="0" />
<xs:element ref="sql-update" minOccurs="0" />
<xs:element ref="sql-delete" minOccurs="0" />
</xs:sequence>
<xs:attribute name="table" use="required" type="xs:string" />
<xs:attribute name="schema" type="xs:string" />
<xs:attribute name="catalog" type="xs:string" />
<xs:attribute name="subselect" type="xs:string" />
<xs:attribute name="fetch" default="join">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="join" />
<xs:enumeration value="select" />
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="inverse" default="false" type="xs:boolean">
</xs:attribute>
<xs:attribute name="optional" default="false" type="xs:boolean">
</xs:attribute>
</xs:complexType>
</xs:element>
As you can see, this allows
the use of "property" child elements or "component" child elements, but not both. It also doesn't allow for "subclass" child elements to continue
the hierarchy below
the point at which
the strategy was changed.
Is there a way to accomplish this?
