Using Caché Objects - InterSystems Documentation

The Persistence Interface
Parameter READONLY = 1;
This is only useful for cases where you have objects that are somehow mapped to preexisting
storage (such as existing globals or an external database). If you call the %Save method on
a READONLY object, it will always return an error code.
12.2.2 Opening Objects
You can open (load an object instance from disk into memory) an object using the %OpenId
method. The %OpenId method takes an ID value as input and returns a reference (OREF)
to an in-memory object or a null value ("") if it cannot find or otherwise open the object.
For example:
// Open person "10"
Set person = ##class(Sample.Person).%OpenId(10)
Write "Person: ",person,!
// should be an object reference
// Open person "-10"
Set person = ##class(Sample.Person).%OpenId(-10)
Write "Person: ",person,!
// should be a null string
Note that in Caché Basic, the OpenId command is equivalent to the %OpenId method:
person = OpenId Sample.Person(1)
PrintLn "Name: " & person.Name
The %OpenId method takes optional second and third arguments. The second argument is
the concurrency level (locking) used to open the object (see below). The third object is a
%Status value, passed by reference, that indicates whether the call succeeded or failed.
The %OpenId method calls the callback method %OnOpen if it is present. Note that the
%OnNew callback is not called when an object instance is opened from the database.
12.2.2.1 Polymorphism and %OpenId
The %OpenId method behaves polymorphically — that is, it may return different types of
objects depending on the ID value it is passed.
For example, the extent (set of) Sample.Person objects includes instances of Sample.Person
as well as instances of Sample.Employee (a subclass of Sample.Person). Calling %OpenId
for the Sample.Person class could return an instance of either Sample.Person or
Sample.Employee depending on what is stored within the database:
Using Caché Objects 103