Introduction to Persistent Objects
This topic presents the concepts that are useful to understand when working with persistent classes.
Some of the examples shown here are from the Samples-Data sample (https://github.com/intersystems/Samples-DataOpens in a new tab). InterSystems recommends that you create a dedicated namespace called SAMPLES (for example) and load samples into that namespace. For the general process, see Downloading Samples for Use with InterSystems IRIS®.
Introduction to the Default SQL Projection
For any persistent class, the compiler generates an SQL table definition, so that the stored data can be accessed via SQL in addition to via the object interface.
The table contains one record for each saved object, and the table can be queried via InterSystems SQL. The following shows the results of a query of the Sample.Person table:
The following table summarizes the default projection:
The Object-SQL Projection
|From (Object Concept) ...
||To (Relational Concept) ...
|Data type property
||Set of fields
Other topics provide details and describe any changes you can make:
Identifiers for Saved Objects: ID and OID
When you save an object for the first time, the system creates two permanent identifiers for it, either of which you can later use to access or remove the saved objects. The more commonly used identifier is the object ID. An ID is a simple literal value that is unique within the table. By default, the system generates an integer to use as an ID.
An OID is more general: it also includes the class name and is unique in the database. In general practice, an application never needs to use the OID value; the ID value is usually sufficient.
The %PersistentOpens in a new tab class provides methods that use either the ID or the OID. You specify an ID when you use methods such as %OpenId(), %ExistsId(), and %DeleteId(). You specify the OID as the argument for methods such as %Open(), %Exists(), and %Delete(). That is, the methods that use ID as an argument include Id in their names. The methods that use OID as the argument do not include Id in their names; these methods are used much less often.
When a persistent object is stored in the database, the values of any of its reference attributes (that is, references to other persistent objects) are stored as OID values. For object attributes that do not have OIDs, the literal value of the object is stored along with the rest of the state of the object.
Projection of Object IDs to SQL
The ID of an object is available in the corresponding SQL table. If possible, InterSystems IRIS uses the field name ID. InterSystems IRIS also provides a way to access the ID if you are not sure what field name to use. The system is as follows:
An object ID is not a property of the object and is treated differently from the properties.
If the class does not contain a property named ID (in any case variation), then the table also contains the field ID, and that field contains the object ID. For an example, see the previous section.
If the class contains a property that is projected to SQL with the name ID (in any case variation), then the table also contains the field ID1, and this field holds the value of the object ID.
Similarly, if the class contains properties that are projected as ID and ID1, then the table also contains the field ID2, and this field holds the value of the object ID.
In all cases, the table also provides the pseudo-field %ID, which holds the value of the object ID.
The OID is not available in the SQL table.
Object IDs in SQL
InterSystems IRIS enforces uniqueness for the ID field (whatever its actual name might be). InterSystems IRIS also prevents this field from being changed. This means that you cannot perform SQL UPDATE or INSERT operations on this field. For instance, the following shows the SQL needed to add a new record to a table:
INSERT INTO PERSON (FNAME, LNAME)VALUES (:fname, :lname)
Notice that this SQL does not refer to the ID field. InterSystems IRIS generates a value for the ID field and inserts that when it creates the requested record.
Other Class Members
A class method or a class query can be defined so that it can be invoked as a stored procedureOpens in a new tab, which enables you to invoke it from SQL.
For class members not discussed in this topic, there is no projection to SQL. That is, InterSystems IRIS does not provide a direct way to use them from SQL or to make them usable from SQL.