Object-relational mapping is the process of mapping persistent classes to relational database tables. In JDOR, you perform object/relational mapping through mapping metadata. Mapping metadata uses XML to describe how to link your object model to your relational model. With a few notable exceptions, every persistent class and every persistent field must have mapping metadata.
![[Note]](img/note.gif) | Note |
|---|---|
Kodo offers tools to automate mapping. See Chapter 7, Mapping in the Reference Guide. |
By default, JDOR implementations expect to find mapping metadata
integrated into the persistence
metadata defined in your .jdo files. You
can, however, instruct the implementation to look for separate mapping
files by supplying a value for the javax.jdo.option.Mapping
key in the Properties used to
construct your PersistenceManagerFactory (see
Section 6.3, “PersistenceManagerFactory Construction”). When this key is
set, JDOR completely ignores any mapping metadata in your
.jdo files. Instead, the implementation looks
for mapping metadata in separate .orm files. The
placement rules for these .orm files are very
similar to the rules for .jdo files: the mapping
document for a persistent class must be available as a resource from
the class' class loader, and must exist in one of two standard
locations:
In a resource called
<class-name>-<mapping>.orm, where<class-name>is the unqualified name of the class the document applies to, and<mapping>is the value of thejavax.jdo.option.Mappingproperty. The resource must be located in the same package as the class.In a resource called
package-<mapping>.orm, where<mapping>is the value of thejavax.jdo.option.Mappingproperty. The resource should be placed in the corresponding package, or in any ancestor package. Package-level documents should contain the mappings for all the persistence-capable classes in the package, except those classes that have individual<class-name>-<mapping>.ormresources associated with them. They may also contain the mappings for classes in any sub-packages.
Assuming you are using a standard Java class loader, these rules
imply that for a javax.jdo.option.Mapping setting of
hsql and a class Magazine
defined by the file org/mag/Magazine.class, the
corresponding mapping document could be defined in any of the following
files:
org/mag/Magazine-hsql.ormorg/mag/package-hsql.ormorg/package-hsql.ormpackage-hsql.orm
Because mapping documents are loaded as resources, JDOR implementations
can also read them from jar files.
| Note |
|---|---|
Kodo offers additional options for mapping metadata placement, including the ability to define mappings yourself at runtime. See Section 7.5, “Mapping Factory”. |
