Using Caché Objects - InterSystems Documentation

12
Object Persistence
One of the most important features of Caché Objects is Object Persistence: the ability to store
and retrieve objects within a database.
Within Caché, object persistence is automatic and built-in: you do not need to write any
persistence code; you do not need to provide any object/relational “mapping” ; and you do
not have to bother with middleware and database connectivity tools.
Every persistent object is automatically projected as an SQL table. This is described in the
Objects and SQL chapter.
Caché provides two flavors of object identity that you can use identify persistent objects: ID
and OID. An ID is a simple literal value (by default, an integer) that is unique within a specific
extent of objects (say the set of all Person objects). An OID is more general: it also includes
the object's class name and is unique across a database of objects. In general practice, an
application never needs to use the OID value; the ID value is usually sufficient. Within this
chapter, we will use the ID variant of the various persistence methods (such as %OpenId)
for simplicity.
12.1 The %Persistent Class
Object persistence is provided by the %Persistent class, which defines the methods of the
Persistence Interface; and the Class Compiler, which manages Schema Evolution and SQL
Projection.
To be persistent, two things must be true about a class definition:
Using Caché Objects 99