The object relationship mapper (ORM) is an ODABA database, which stores extraneous information that is not available in a relational database management system. In order to support relational storage types, the ORM becomes necessary, since instance metadata required is not available in an RDBMS. Moreover, collection metadata (e.g. the modification count for the children collection of person Paul Miller) is required, which is also not available in an RDBMS.
The ORM acts as bridge between an ODABA application and a relational or XML data storage.