Working with Persistent Objects

The %Persistent class is the API for objects that can be saved (written to disk). This chapter describes how to use this API. Information in this chapter applies to all subclasses of %Persistent.

Also see the chapters “Introduction to Persistent Objects,” “Defining Persistent Classes,” and “Other Options for Persistent Classes.”

Most of the samples shown in this chapter are from the Samples-Data sample (https://github.com/intersystems/Samples-Data). 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®.

Saving Objects

To save an object to the database, invoke its %Save() method. For example:

 Set obj = ##class(MyApp.MyClass).%New()
 Set obj.MyValue = 10

 Set sc = obj.%Save()

The %Save() method returns a %Status value that indicates whether the save operation succeeded or failed. Failure could occur, for example, if an object has invalid property values or violates a uniqueness constraint; see “Validating Objects” in the previous chapter.

Calling %Save() on an object automatically saves all modified objects that can be “reached” from the object being saved: that is, all embedded objects, collections, streams, referenced objects, and relationships involving the object are automatically saved if needed. The entire save operation is carried out as one transaction: if any object fails to save, the entire transaction fails and rolls back (no changes are made to disk; all in-memory object values are what they were before calling %Save()).

When an object is saved for the first time, the default behavior is for the %Save() method to automatically assign it an object ID value that is used to later find the object within the database. In the default case, the ID is generated using the $Increment function; alternately, the class can use a user-provided object ID based on property values that have an idkey index (and, in this case, the property values cannot include the string “||”) . Once assigned, you cannot alter the object ID value for a specific object instance (even if it is a user-provided ID).

You can find the object ID value for an object that has been saved using the %Id() method:

 // Open person "22"
 Set person = ##class(Sample.Person).%OpenId(22)
 Write "Object ID: ",person.%Id(),!

In more detail, the %Save() method does the following:

First it constructs a temporary structure known as a “SaveSet.” The SaveSet is simply a graph containing references to every object that is reachable from the object being saved. (Generally, when an object class A has a property whose value is another object class B, an instance of A can “reach” an instance of B.) The purpose of the SaveSet is to make sure that save operations involving complex sets of related objects are handled as efficiently as possible. The SaveSet also resolves any save order dependencies among objects.
As each object is added to the SaveSet, its %OnAddToSaveSet() callback method is called, if present.
It then visits each object in the SaveSet in order and checks if they are modified (that is, if any of their property values have been modified since the object was opened or last saved). If an object has been modified, it will then be saved.
Before being saved, each modified object is validated (its property values are tested; its %OnValidateObject() method, if present, is called; and then its uniqueness constraints are tested); if the object is valid, the save proceeds. If any object is invalid, then the call to %Save() fails and the current transaction is rolled back.
Before and after saving each object, the %OnBeforeSave() and %OnAfterSave() callback methods are called, if present. Similarly, any defined BEFORE INSERT, BEFORE UPDATE, AFTER INSERT, and AFTER UPDATE triggers are called as well.
These callbacks and triggers are passed an Insert argument, which indicates whether an object is being inserted (saved for the first time) or updated.
If one of these callback methods or triggers fails (returns an error code) then the call to %Save() fails and the current transaction is rolled back.
Lastly, the %OnSaveFinally() callback method is called, if present.
This callback method is called after the transaction has completed and the status of the save has been finalized.

If the current object is not modified, then %Save() does not write it to disk; it returns success because the object did not need to be saved and, therefore, there is no way that there could have been a failure to save it. In fact, the return value of %Save() indicates that the save operation either did all that it was asked or it was unable to do as it was asked (and not specifically whether or not anything was written to disk).

Important:

In a multi-process environment, be sure to use proper concurrency controls; see “Object Concurrency Options.”

Rollback

The %Save() method automatically saves all the objects in its SaveSet as a single transaction. If any of these objects fail to save, then the entire transaction is rolled back. In this rollback, InterSystems IRIS does the following:

It reverts assigned IDs.
It recovers removed IDs.
It recovers modified bits.
It invokes the %OnRollBack() callback method, if implemented, for any object that had been successfully serialized.
InterSystems IRIS does not invoke this method for an object that has not been successfully serialized, that is, an object that is not valid.

Saving Objects and Transactions

As noted previously, the %Save() method automatically saves all the objects in its SaveSet as a single transaction. If any of these objects fail to save, then the entire transaction is rolled back.

If you wish to save two or more unrelated objects as a single transaction, however, you must enclose the calls to %Save() within an explicit transaction: that is, you must start the transaction using the TSTART command and end it with the TCOMMIT command.

For example:

 // start a transaction
 TSTART

 // save first object
 Set sc = obj1.%Save()

 // save second object (if first was save)
 If ($$$ISOK(sc)) {
     Set sc = obj2.%Save()
 }

 // if both saves are ok, commit the transaction
 If ($$$ISOK(sc)) {
     TCOMMIT
 }

There are two things to note about this example:

The %Save() method knows if it is being called within an enclosing transaction (because the system variable, $TLEVEL, will be greater than 0).
If any of the %Save() methods within the transaction fails, the entire transaction is rolled back (the TROLLBACK command is invoked). This means that an application must test every call to %Save() within a explicit transaction and if one fails, skip calling %Save() on the other objects and skip invoking the final TCOMMIT command.

Testing the Existence of Saved Objects

There are two basic ways to test if a specific object instance is stored within the database:

In these examples, the ID is an integer, which is how InterSystems IRIS generates IDs by default. The next chapter describes how you can define a class so that the ID is instead based on a unique property of the object.

Testing for Object Existence with ObjectScript

The %ExistsId() class method checks a specified ID; it returns a true value (1) if the specified object is present in the database and false (0) otherwise. It is available to all classes that inherit from %Persistent. For example:

 Write ##class(Sample.Person).%ExistsId(1),!   // should be 1 
 Write ##class(Sample.Person).%ExistsId(-1),!  // should be 0

Here, the first line should return 1 because Sample.Person inherits from %Persistent and the SAMPLES database provides data for this class.

You can also use the %Exists() method, which requires an OID rather than an ID. An OID is a permanent identifier unique to the database that includes both the ID and the class name of the object. For more details, see Identifiers for Saved Objects: ID and OID.

Testing for Object Existence with SQL

To test for the existence of a saved object with SQL, use a SELECT statement that selects a row whose %ID field matches the given ID. (The identity property of a saved object is projected as the %ID field.)

For example, using embedded SQL:

 &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

Here, the first case should result in an SQLCODE of 0 (meaning success) because Sample.Person inherits from %Persistent and the SAMPLES database provides data for this class.

The second case should result in an SQLCODE of 100, which means that the statement successfully executed but returned no data. This is expected because the system never automatically generates an ID value less than zero.

For more information on embedded SQL, see the chapter “Embedded SQL” in Using InterSystems SQL. For more information on SQLCODE, see “SQLCODE Values and Error Messages” in the InterSystems IRIS Error Reference.

Opening Saved Objects

To open an object (load an object instance from disk into memory), use the %OpenId() method, which is as follows:

classmethod %OpenId(id As %String, 
                    concurrency As %Integer = -1, 
                    ByRef sc As %Status = $$$OK) as %ObjectHandle

Where:

id is the ID of the object to open.
In these examples, the ID is an integer. The next chapter describes how you can define a class so that the ID is instead based on a unique property of the object.
concurrency is the concurrency level (locking) used to open the object.
sc, which is passed by reference, is a %Status value that indicates whether the call succeeded or failed.

The method returns an OREF if it can open the given object. It returns 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

You can also use the %Open() method, which requires an OID rather than an ID. An OID is a permanent identifier unique to the database that includes both the ID and the class name of the object. For more details, see Identifiers for Saved Objects: ID and OID.

To perform additional processing that executes when an object opens, you can define the %OnOpen() and %OnOpenFinally() callback methods. For more details, see Defining Callback Methods.

Multiple Calls to %OpenId()

If %OpenId() is called multiple times within an InterSystems IRIS process for the same ID and the same object, only one object instance is created in memory. All subsequent calls to %OpenId() return a reference to the object already loaded into memory.

For example, consider this class featuring multiple %OpenId() calls to the same object:

Class User.TestOpenId Extends %RegisteredObject
{

ClassMethod Main()
{
   set A = ##class(Sample.Person).%OpenId(1)
   write "A: ", A.Name

   set A.Name = "David"
   write !, "A after set: ", A.Name

   set B = ##class(Sample.Person).%OpenId(1)
   write !, "B: ", B.Name

   do ..Sub()

   job ..JobSub()
   hang 1
   write !, "D in JobSub: ", ^JobSubD
   kill ^JobSubD

   kill A, B
   set E = ##class(Sample.Person).%OpenId(1)
   write !, "E:", E.Name
}

ClassMethod Sub()
{
   set C = ##class(Sample.Person).%OpenId(1)
   write !, "C in Sub: ", C.Name
}

ClassMethod JobSub()
{
   set D = ##class(Sample.Person).%OpenId(1)
   set ^JobSubD = D.Name
}

}

Calling the Main() method produces these results:

The initial %OpenId() call loads the instance object into memory. The A variable references this object and you can modify the loaded object through this variable.
```
   set A = ##class(Sample.Person).%OpenId(1)
   write "A: ", A.Name
```
A: Uhles,Norbert F.
```
   set A.Name = "David"
   write !, "A after set: ", A.Name
```
A after set: David
The second %OpenId() call does not reload the object from the database and does not overwrite the changes made using variable A. Instead, variable B references the same object as variable A.
```
   set B = ##class(Sample.Person).%OpenId(1)
   write !, "B: ", B.Name
```
B: David
The third %OpenId() call, in the Sub() method, also references the previously loaded object. This method is part of the same InterSystems IRIS process as the Main() method. Variable C references the same object as variables A and B, even though those variables are not available within the scope of this method. At the end of the Sub() method, the process returns back to the Main() method, the variable C is destroyed, and variables A and B are back in scope.
```
   do ..Sub()
```
```
ClassMethod Sub()
{
   set C = ##class(Sample.Person).%OpenId(1)
   write !, "C in Sub: ", C.Name
}
```
C in Sub: David
The fourth %OpenId() call, in the JobSub() method, is run in a separate InterSystems IRIS process by using the JOB command. This new process loads a new instance object into memory, and variable D references this new object.
```
   job ..JobSub()
   hang 1
   write !, "D in JobSub: ", ^JobSubD
   kill ^JobSubD
```
```
ClassMethod JobSub()
{
   set D = ##class(Sample.Person).%OpenId(1)
   set ^JobSubD = D.Name
}
```
D in JobSub: Uhles,Norbert F.
The fifth %OpenId() call occurs after deleting all variables (A and B) that reference the loaded object in the original process, thereby removing that object from memory. As with the previous %OpenId() call, this call also loads a new instance object into memory, and variable E references this object.
```
   kill A, B
   set E = ##class(Sample.Person).%OpenId(1)
   write !, "E:", E.Name
```
E: Uhles,Norbert F.

To force a reload of an object being loaded multiple times, you can use the %Reload() method. Reloading the object reverts any changes made to the object that were not saved. Any previously assigned variables reference the reloaded version. For example:

 set A = ##class(Sample.Person).%OpenId(1)
 write "A: ", A.Name // A: Uhles,Norbert F.
 
 set A.Name = "David"
 write !, "A after set: ", A.Name // David
 
 set B = ##class(Sample.Person).%OpenId(1)
 write !, "B before reload: ", B.Name // David

 do B.%Reload()
 write !, "B after reload: ", B.Name // Uhles,Norbert F.
 write !, "A after reload: ", A.Name // Uhles,Norbert F.

Concurrency

The %OpenId() method takes an optional concurrency argument as input. This argument specifies the concurrency level (type of locks) that should be used to open the object instance.

If the %OpenId() method is unable to acquire a lock on an object, it fails.

To raise or lower the current concurrency setting for an object, reopen it with %OpenId() and specify a different concurrency level. For example,

 set person = ##class(Sample.Person).%OpenId(6,0)

opens person with a concurrency of 0 and the following effectively upgrades the concurrency to 4:

 set person = ##class(Sample.Person).%OpenId(6,4)

Specifying a concurrency level of 3 (shared, retained lock) or 4 (shared, exclusive lock) forces a reload of the object when opening it. For more information on the possible object concurrency levels, see Object Concurrency Options.

Swizzling

If you open (load into memory) an instance of a persistent object, and use an object that it references, then this referenced object is automatically opened. This process is referred to as swizzling; it is also sometimes known as “lazy loading.”

For example, the following code opens an instance of Sample.Employee object and automatically swizzles (opens) its related Sample.Company object by referring to it using dot syntax:

 // Open employee "101"
 Set emp = ##class(Sample.Employee).%OpenId(101)

 // Automatically open Sample.Company by referring to it:
 Write "Company: ",emp.Company.Name,!

When an object is swizzled, it is opened using the default concurrency value of the class it is a member of, not the concurrency value of the object that swizzles it. See “Object Concurrency Options,” later in this chapter.

A swizzled object is removed from memory as soon as no objects or variables refer to it.

Reading Stored Values

Suppose you have opened an instance of a persistent object, modified its properties, and then wish to view the original value stored in the database before saving the object. The easiest way to do this is to use an SQL statement (SQL is always executed against the database; not against objects in memory).

For example:

 // Open person "1"
 Set person = ##class(Sample.Person).%OpenId(1)
 Write "Original value: ",person.Name,!

 // modify the object
 Set person.Name = "Black,Jimmy Carl"
 Write "Modified value: ",person.Name,!

 // Now see what value is on disk
 Set id = person.%Id()
 &sql(SELECT Name INTO :name
         FROM Sample.Person WHERE %ID = :id)

 Write "Disk value: ",name,!

Deleting Saved Objects

The persistence interface includes methods for deleting objects from the database.

The %DeleteId() Method

The %DeleteId() method deletes an object that is stored within a database, including any stream data associated with the object. This method is as follows:

classmethod %DeleteId(id As %String, concurrency As %Integer = -1) as %Status

Where:

id is the of the object to open.
In these examples, the ID is an integer. The next chapter describes how you can define a class so that the ID is instead based on a unique property of the object.
concurrency is the concurrency level (locking) used when deleting the object.

For example:

 Set sc = ##class(MyApp.MyClass).%DeleteId(id)

%DeleteId() returns a %Status value indicating whether the object was deleted or not.

%DeleteId() calls the %OnDelete() callback method (if present) before deleting the object. %OnDelete() returns a %Status value; if %OnDelete() returns an error value, then the object will not be deleted, the current transaction is rolled back, and %DeleteId() returns an error value.

%DeleteId() calls the %OnAfterDelete() callback method (if present) after deleting the object. This call occurs immediately after %DeleteData() is called, provided that %DeleteData() does not return an error. If %OnAfterDelete() returns an error, then the current transaction is rolled back, and %DeleteId() returns an error value.

Lastly, %DeleteId() calls the %OnDeleteFinally() callback method, if present. This callback method is called after the transaction has completed and the status of the deletion has been finalized.

Note that the %DeleteId() method has no effect on any object instances in memory.

You can also use the %Delete() method, which requires an OID rather than an ID. An OID is a permanent identifier unique to the database that includes both the ID and the class name of the object. For more details, see Identifiers for Saved Objects: ID and OID.

The %DeleteExtent() Method

The %DeleteExtent() method deletes every object (and subclass of object) within its extent. Specifically it iterates through the entire extent and invokes the %DeleteId() method on each instance.

The %KillExtent() Method

The %KillExtent() method directly deletes the globals that store an extent of objects (not including data associated with streams). It does not invoke the %DeleteId() method and performs no referential integrity actions. This method is simply intended to serve as a help to developers during the development process. (It is similar to the TRUNCATE TABLE command found in older relational database products.) If you need to delete every object in an extent, including associated stream data, use %DeleteExtent() instead.

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.

Accessing Object Identifiers

If an object has been saved, it has an ID and an OID, the permanent identifiers that are used on disk. If you have an OREF for the object, you can use that to obtain these identifiers.

To find the ID associated with an OREF, call the %Id() method of the object. For example:

 write oref.%Id()

To find the OID associated with an OREF, you have two options:

Call the %Oid() method of the object. For example:
```
 write oref.%Oid()
```
Access the %%OID property of the object. Because this property name contains % characters, you must enclose the name in double quotes. For example:
```
 write oref."%%OID"
```

Object Concurrency Options

It is important to specify concurrency appropriately when you open or delete objects. You can specify concurrency at several different levels:

You can specify the concurrency argument for the method that you are using.
Many of the methods of the %Persistent class allow you to specify this argument, an integer. This argument determines how locks are used for concurrency control. A later subsection lists the allowed concurrency values.
If you do not specify the concurrency argument in a call to the method, the method sets the argument to -1, which means that InterSystems IRIS uses the value of the DEFAULTCONCURRENCY class parameter for the class you are working with; see the next item.
You can specify the DEFAULTCONCURRENCY class parameter for the associated class. All persistent classes inherit this parameter from %Persistent, which defines the parameter as an expression that obtains the default concurrency for the process; see the next item.
You could override this parameter in your class and specify a hardcoded value or an expression that determines the concurrency via your own rules. In either case, the value of the parameter must be one of the allowed concurrency values discussed later in this section.
You can set the default concurrency mode for a process. To do so, use the $system.OBJ.SetConcurrencyMode() method (which is the SetConcurrencyMode() method of the %SYSTEM.OBJ class).
As in the other cases, you must use one of the allowed concurrency values. If you do not set the concurrency mode for a process explicitly, the default value is 1.
The $system.OBJ.SetConcurrencyMode() method has no effect on any classes that specify an explicit value for the DEFAULTCONCURRENCY class parameter.

To take a simple example, if you open a persistent object using its %OpenId() method without specifying a concurrency value (or you specify a value of -1 explicitly), the class does not specify the DEFAULTCONCURRENCY class parameter, and you do not set the concurrency mode in code, the default concurrency value for the object is set to 1.

Note:

Some system classes have the DEFAULTCONCURRENCY class parameter set to a value of 0. Sharded classes always use a concurrency value of 0. To check the concurrency value of an object, examine its %Concurrency property.

Why Specify Concurrency?

The following scenario demonstrates why it is important to control concurrency appropriately when you read or write objects. Consider the following scenario:

Process A opens an object without specifying the concurrency.

SAMPLES>set person = ##class(Sample.Person).%OpenId(5)
 
SAMPLES>write person
1@Sample.Person

Process B opens the same object with the concurrency value of 4.

SAMPLES>set person = ##class(Sample.Person).%OpenId(5, 4)
 
SAMPLES>write person
1@Sample.Person

Process A modifies a property of the object and attempts to save it using %Save() and receives an error status.

SAMPLES>do person.FavoriteColors.Insert("Green")

SAMPLES>set status = person.%Save()
 
SAMPLES>do $system.Status.DisplayError(status)
 
ERROR #5803: Failed to acquire exclusive lock on instance of 'Sample.Person'

This is an example of concurrent operations without adequate concurrency control. For example, if process A could possibly save the object back to the disk, it must open the object with concurrency 4 to ensure it can save the object without conflict with other processes. (These values are discussed later in this chapter.) In this case, Process B would then be denied access (failed with a concurrency violation) or would have to wait until Process A releases the object.

Concurrency Values

This section describes the possible concurrency values. First, note the following details:

Atomic writes are guaranteed when concurrency is greater than 0.
InterSystems IRIS acquires and releases locks during operations such as saving and deleting objects; the details depend upon the concurrency value, what constraints are present, lock escalation status, and the storage structure.
In all cases, when an object is removed from memory, any locks for it are removed.

The possible concurrency values are as follows; each value has a name, also shown in the list.

Concurrency Value 0 (No locking)

No locks are used.

Concurrency Value 1 (Atomic read)

Locks are acquired and released as needed to guarantee that an object read will be executed as an atomic operation.

InterSystems IRIS does not acquire any lock when creating a new object.

While opening an object, InterSystems IRIS acquires a shared lock for the object, if that is necessary to guarantee an atomic read. InterSystems IRIS releases the lock after completing the read operation.

The following table lists the locks that are present in each scenario:

	When object is created	While object is being opened	After object has been opened	After save operation is complete
New object	no lock	N/A	N/A	no lock
Existing object	N/A	shared lock, if that is necessary to guarantee an atomic read	no lock	no lock

Concurrency Value 2 (Shared locks)

The same as 1 (atomic read) except that opening an object always acquires a shared lock (even if the lock is not needed to guarantee an atomic read). The following table lists the locks that are present in each scenario:

	When object is created	While object is being opened	After object has been opened	After save operation is complete
New object	no lock	N/A	N/A	no lock
Existing object	N/A	shared lock	no lock	no lock

Concurrency Value 3 (Shared/retained locks)

InterSystems IRIS does not acquire any lock when creating a new object.

While opening an existing object, InterSystems IRIS acquires a shared lock for the object.

After saving a new object, InterSystems IRIS has a shared lock for the object.

The following table lists the scenarios:

	When object is created	While object is being opened	After object has been opened	After save operation is complete
New object	no lock	N/A	N/A	shared lock
Existing object	N/A	shared lock	shared lock	shared lock

Concurrency Value 4 (Exclusive/retained locks)

When an existing object is opened or when a new object is first saved, InterSystems IRIS acquires an exclusive lock.

The following table lists the scenarios:

	When object is created	While object is being opened	After object has been opened	After save operation is complete
New object	no lock	N/A	N/A	exclusive lock
Existing object	N/A	exclusive lock	exclusive lock	exclusive lock

Concurrency and Swizzled Objects

An object referenced by a property is swizzled on access using the default concurrency defined by the swizzled object’s class. If the default is not defined for the class, the object is swizzled using the default concurrency mode of the process. The swizzled object does not use the concurrency value of the object that swizzles it.

If the object being swizzled is already in memory, then swizzling does not actually open the object — it simply references the existing in-memory object; in that case, the current state of the object is maintained and the concurrency is unchanged.

There are two ways to override this default behavior:

Upgrade the concurrency on the swizzled object with a call to the %Open() method that specifies the new concurrency. For example:
```
 Do person.Spouse.%OpenId(person.Spouse.%Id(),4,.status) 
```
where the first argument to %OpenId() specifies the ID, the second specifies the new concurrency, and the third (passed by reference) receives the status of the method.
Set the default concurrency mode for the process before swizzling the object. For example:
```
 Set olddefault = $system.OBJ.SetConcurrencyMode(4) 
```
This method takes the new concurrency mode as its argument and returns the previous concurrency mode.
When you no longer need a different concurrency mode, reset the default concurrency mode as follows:
```
 Do $system.OBJ.SetConcurrencyMode(olddefault)
```

Version Checking (Alternative to Concurrency Argument)

Rather than specifying the concurrency argument when you open or delete an object, you can implement version checking. To do so, you specify a class parameter called VERSIONPROPERTY. All persistent classes have this parameter. When defining a persistent class, the procedure for enabling version checking is:

Create a property of type %Integer that holds the updateable version number for each instance of the class.
For that property, set the value of the InitialExpression keyword to 0.
For the class, set the value of the VERSIONPROPERTY class parameter equal to the name of that property. The value of VERSIONPROPERTY cannot be changed to a different property by a subclass.

This incorporates version checking into updates to instances of the class.

When version checking is implemented, the property specified by VERSIONPROPERTY is automatically incremented each time an instance of the class is updated (either by objects or SQL). Prior to incrementing the property, InterSystems IRIS compares its in-memory value to its stored value. If they are different, then a concurrency conflict is indicated and an error is returned; if they are the same, then the property is incremented and saved.

Note:

You can use this set of features to implement optimistic concurrency.

To implement a concurrency check in an SQL update statement for a class where VERSIONPROPERTY refers to a property called InstanceVersion, the code would be something like:

 SELECT InstanceVersion,Name,SpecialRelevantField,%ID
     FROM my_table
     WHERE %ID = :myid

     // Application performs operations on the selected row

 UPDATE my_table
     SET SpecialRelevantField = :newMoreSpecialValue
     WHERE %ID = :myid AND InstanceVersion = :myversion

where myversion is the value of the version property selected with the original data.

Defining Persistent Classes

Introduction to Persistent Objects

Working with Persistent Objects

Copy link to this sectionSaving Objects

Copy link to this sectionRollback

Copy link to this sectionSaving Objects and Transactions

Copy link to this sectionTesting the Existence of Saved Objects

Copy link to this sectionTesting for Object Existence with ObjectScript

Copy link to this sectionTesting for Object Existence with SQL

Copy link to this sectionOpening Saved Objects

Copy link to this sectionMultiple Calls to %OpenId()

Copy link to this sectionConcurrency

Copy link to this sectionSwizzling

Copy link to this sectionReading Stored Values

Copy link to this sectionDeleting Saved Objects

Copy link to this sectionThe %DeleteId() Method

Copy link to this sectionThe %DeleteExtent() Method

Copy link to this sectionThe %KillExtent() Method

Copy link to this sectionAccessing Object Identifiers

Copy link to this sectionObject Concurrency Options

Copy link to this sectionWhy Specify Concurrency?

Copy link to this sectionConcurrency Values

Copy link to this sectionConcurrency and Swizzled Objects

Copy link to this sectionVersion Checking (Alternative to Concurrency Argument)

Saving Objects

Rollback

Saving Objects and Transactions

Testing the Existence of Saved Objects

Testing for Object Existence with ObjectScript

Testing for Object Existence with SQL

Opening Saved Objects

Multiple Calls to %OpenId()

Concurrency

Swizzling

Reading Stored Values

Deleting Saved Objects

The %DeleteId() Method

The %DeleteExtent() Method

The %KillExtent() Method

Accessing Object Identifiers

Object Concurrency Options

Why Specify Concurrency?

Concurrency Values

Concurrency and Swizzled Objects

Version Checking (Alternative to Concurrency Argument)