Using Caché Objects - InterSystems Documentation

Object Persistence
If a persistent class, B, is a subclass of another persistent class, A, then all of the stored
instances of B (as well as subclasses of B) belong to the superclass extent A as well as to the
subextent B.
The top-most class in a hierarchy of persistent classes (that is, the first superclass marked as
persistent) is referred to as a root class and defines the top-level extent. The root class defines
the primary storage used by all objects stored within the extent. If the root class specifies that
its instances will be stored within the global ^MyApp.Data, then all subclasses in this extent
are also stored there. Note that the root class of an extent can be marked as Abstract meaning
that there are no actual instances of this class stored within its extent.
An extent that includes objects of different subtypes is referred to as a multi-class extent.
Note that as far as persistence is concerned only the primary (first) superclass of an object
determines its type and what extent it belongs to.
Suppose we define the following persistent objects:
• MyApp.Person — a persistent class (directly derived from %Persistent).
• MyApp.Teacher — a persistent class derived from MyApp.Person.
• MyApp.Student — a persistent class derived from MyApp.Person.
• MyApp.GradStudent — a persistent class derived from MyApp.Student.
In this case, the root class is MyApp.Person as it is directly derived from %Persistent.
MyApp.Person defines the top-level extent MyApp.Person.
MyApp.Teacher defines the subextent MyApp.Teacher. All instances of MyApp.Teacher belong
the to top-level MyApp.Person extent as well as the MyApp.Teacher subextent.
Similarly, MyApp.Student defines the subextent MyApp.Student. All instances of MyApp.Student,
as well as its subclass MyApp.GradStudent, belong the to top-level MyApp.Person extent as
well as the MyApp.Student subextent.
MyApp.GradStudent is a subclass of MyApp.Student. It defines the subextent
MyApp.GradStudent. All instances of MyApp.GradStudent belong the to top-level MyApp.Person
extent, the MyApp.Student subextent, and the MyApp.GradStudent subextent.
12.3.1 The Extent Query
Every persistent class automatically includes a class query called Extent that provides a set
of all the object ID values within an object extent.
108 Using Caché Objects