How to: Restore Deleted Objects

• Oct 29, 2020
• 3 minutes to read

Persistent objects, that are XPCustomObject or XPObject descendants, are not physically deleted from a database. Instead, these objects are only marked as deleted. XPO creates a GCRecord column in a database table for a given persistent class. When an object is deleted, a not-NULL value is assigned to the corresponding GCRecord field value. This is the Deferred Deletion feature, which enables the avoidance of constraint violations when deleting associated objects.

This example demonstrates how to restore deleted objects. To do this, a null (Nothing in VB.NET) value should be assigned to the corresponding GCRecord field value via the XPBaseObject.SetMemberValue method.

Note

When an object with Deferred Deletion enabled is deleted, it is removed from associated collections. Associations cannot be recreated automatically when you restore a deleted object. This behavior is by design, because the Deferred Deletion feature is not intended for object restoration, but for overcoming complex foreign index constraints at the database level.

Persistent Object Class Definition

First, define the Customer class. To provide for the automatic collection of changes by a Unit of Work, Simplified Property Syntax is used to code property setters.

C#

class Customer : XPObject {
    string fName;
    int fAge;

    public Customer(Session session) : base(session) { }   

    public string Name {
        get { return fName; }
        set { SetPropertyValue(nameof(Name), ref fName, value); }
    }

    public int Age {
        get { return fAge; }
        set { SetPropertyValue(nameof(Age), ref fAge, value); }
    }   
}

VB.NET

Friend Class Customer
    Inherits XPObject
    Private fName As String
    Private fAge As Integer

    Public Sub New(ByVal session As Session)
        MyBase.New(session)
    End Sub

    Public Property Name() As String
        Get
            Return fName
        End Get
        Set(ByVal value As String)
            SetPropertyValue(NameOf(Name), fName, value)
        End Set
    End Property

    Public Property Age() As Integer
        Get
            Return fAge
        End Get
        Set(ByVal value As Integer)
            SetPropertyValue(NameOf(Age), fAge, value)
        End Set
    End Property
End Class

Restoring Deleted Objects

C#

using System.Collections;
using DevExpress.Xpo;

private void RestoreDeletedCustomers(Session session) {
    using (NestedUnitOfWork restore = session.BeginNestedUnitOfWork()) {
        ICollection delCol = restore.GetObjects(restore.GetClassInfo<Customer>(),
            Customer.Fields.GCRecord.IsNotNull(), null, 0, true, true);
        if (delCol != null && delCol.Count != 0) {
            foreach (Customer cust in delCol) {
                cust.SetMemberValue(Customer.Fields.GCRecord.PropertyName, null);
            }
            restore.CommitChanges();
        }
    }
}

VB.NET

Imports System.Collections
Imports DevExpress.Xpo

Private Sub RestoreDeletedCustomers(ByVal session As Session)
    Using restore As NestedUnitOfWork = session.BeginNestedUnitOfWork()
        Dim delCol As ICollection = restore.GetObjects(restore.GetClassInfo(Of Customer)(), _
            Customer.Fields.GCRecord.IsNotNull(), Nothing, 0, True, True)
        If delCol IsNot Nothing AndAlso delCol.Count <> 0 Then
            For Each cust As Customer In delCol
                cust.SetMemberValue(Customer.Fields.GCRecord.PropertyName, Nothing)
            Next cust
            restore.CommitChanges()
        End If
    End Using
End Sub