Using Caché Objects - InterSystems Documentation

Object Extents
(It is similar to the TRUNCATE TABLE command found in older relational database products).
CAUTION:
%KillExtent is intended for use only in a development environment and
should not be used in a live application. %KillExtent bypasses constraints
and user-implemented callbacks, potentially causing data integrity problems.
12.2.4 Testing if Objects Exist
There are two basic ways with which you can test if a specific object instance is stored within
the database.
The first way is to use the %ExistsId method provided by the %Persistent class. This is a
class method that takes an ID value and returns a true value (1) if the specified object is
present in the database and false (0) otherwise. For example:
Write ##class(Sample.Person).%ExistsId(1),! // should be 1
Write ##class(Sample.Person).%ExistsId(-1),! // should be 0
The other way is to use an SQL statement and test the value of SQLCODE:
&sql(SELECT %ID FROM Sample.Person WHERE %ID = '1')
Write SQLCODE,! // should be 0: success
&sql(SELECT %ID FROM Sample.Person WHERE %ID = '-1')
Write SQLCODE,! // should be 100: not found
12.3 Object Extents
A set of object instances of a similar type (class) stored within the database is referred to as
an extent.
For a single persistent class (with no subclasses), an extent is the same as a table in relational
database terms. For example, consider the persistent class MyApp.MyPerson:
Class MyApp.MyPerson Extends %Persistent [classtype = persistent]
{
Property Name As %String;
}
The set of all instances of MyApp.MyPerson stored within the database is the MyApp.MyPerson
extent. This is equivalent to a MyApp.MyPerson table with a Name column.
Extents become more interesting when we consider subclasses. An object extent includes all
instances of itself and any of its subclasses.
Using Caché Objects 107