TechTip: Cascading Deletes : Level 200 DevForce Express View...

Mon, 09 Jul 2007 12:12:24 -700

Author: IdeaBlade
Subject: 115
Posted: 09-Jul-2007 at 12:12pm

Cascading Deletes

Suppose we have a certain type of entity with related children. When we delete an instance of that entity, we want all of its children to be deleted as well. What's the best approach to implementing this capability in DevForce? When performing operations on the graph of an object, we need to keep in mind the potential difference between how that object graph appears in the PersistenceManager's local cache, and how it may appear in the data source. Suppose our object graph is for an Order object, and that the related objects we want to delete consist solely and entirely of the related OrderDetail objects. Assume, for a moment, the following: We will access the OrderDetails of the Order to be deleted through the Order object's OrderDetails relation property; and We did not prefetch any OrderDetails In that case, the OrderDetails in the cache for Order A will be those that existed at the time we first referenced that OrderDetails property in our code. If we have not referenced it prior to attempting the delete on Order A, it will be referenced for the first time in whatever method we provide to do the deletion. But either way, additional time will pass between the moment when we delete the Order and its OrderDetails and the moment when we commit those deletions to the datasource using a SaveChanges(). Unless we have some locking scheme in place to prevent other users from adding OrderDetails to Order A, the possibility exists that they will do so, and that we will still end up with orphaned OrderDetail records in the database. If we don't want that to happen, then we have two choices:
1.	Implement a locking scheme;
2.	Implement a trigger on the database that deletes the OrderDetail records when their Order record is deleted.
3.	Set the value of the Tag property for one of the RadioButtons to one of the possible values of the targetted business object property. Do the same for the remaining RadioButtons, until each possible value that the business object property can take on is represented by exactly one RadioButton.
All right, suppose we've done one of the preceding, or that we're willing to live with an occasional orphaned record in the database.* How can we implement the deletion of the OrderDetails? There are three basic options (short of writing ad hoc code everywhere we want the operation to occur):
1.	Override the Delete() method in the Order class
2.	Provide a separate method - let's call in DeleteGraph() - in the Order class to do the job, leaving the Delete() method alone; or
3.	Provide some other object to watch the PersistenceManager for Order deletions and step in to delete the related OrderDetails whenever such a deletion is requested.
Overriding the Delete() Method This option is the one that comes to mind first for many developers. However, prior to version 3.5.0 of DevForce, it was a pseudo-option, because the Delete() method in a DevForce business class was inherited from System.Data.DataRow, where it is marked as sealed (non-overrideable). But, beginning with version 3.5.0, the Entity class contains a Delete() method which shadows System.Data.DataRow, is marked as virtual, and can thus be overridden.
		* the incidence of which we could minimize by always doing a DataSourceOnly retrieval of the OrderDetail records just before marking them for deletion; and always calling SaveChanges() immediately after so marking them.
Here's a Delete() override that addresses the use case we proposed above for deletion of an Order and its related OrderDetails:

C#:

public override void Delete() {
  EntityList orderDetails =
    new EntityList(this.OrderDetails);
  int orderDetailsCount = orderDetails.Count;
  for (int counter = 0; counter < orderDetailsCount; counter++) {
   orderDetails[0].Delete();
  }
  base.Delete();
}

VB.NET:

PublicOverridesSub Delete()
  Dim orderDetails As EntityList(Of OrderDetail) = New EntityList(Of
OrderDetail)(Me.OrderDetails)
  Dim orderDetailsCount AsInteger = orderDetails.Count
  Dim counter AsInteger = 0
  DoWhile counter < orderDetailsCount
    orderDetails(0).Delete()
     counter += 1
  Loop
  MyBase.Delete()
EndSub

Calling Delete() on an Entity Cast to a DataRow

Since Entity.Delete() shadows rather than overrides DataRow.Delete(), the developer must be aware that calling Delete() on a business object that has been cast to a DataRow will produce a different result than calling Delete() directly on the business object (in the case where Delete() has been overridden in the developer class). The former will only delete the specified DataRow itself, and will perform none of the supplementary actions coded into the Delete() override.

You could, for example, substitute for the following line from the above method:

C#:

base.Delete();

VB.NET:

MYbase.Delete();

this one:

C#:

((DataRow).Delete();

VB.NET:

CType(Me, DataRow).Delete()

Neither statement results in a recursive method call, since the referenced method, in both cases, is a different one from the one in which the statement resides. Providing a DeleteGraph() Method In this option you leave the Order class's Delete() method alone and provide a separate method to do your cascading deletes. We'll say here, for convenience, that we name this method DeleteGraph(), but you can of course call it whatever you like. Whatever its name, its operation will be as follows:
1.	Delete all desired related records, then
2.	Delete the Order object itself
Such a method might look like the following (assuming for simplicity, that the OrderDetail objects are the only related objects to be deleted):

C#:

public int DeleteGraph() {
  int numberOfItemsDeleted = 0;
  EntityList<OrderDetail> orderDetails =
    newEntityList<OrderDetail>(this.OrderDetails);
  int orderDetailsCount = orderDetails.Count;
  for (int counter = 0; counter < orderDetailsCount;counter++){
    orderDetails[0].Delete();
  }
  this.Delete();
  return (orderDetailsCount + 1);
}

VB.NET:

Public Function DeleteGraph() As Integer
  Dim numberOfItemsDeleted As Integer = 0
  Dim orderDetails As EntityList(Of OrderDetail) = New EntityList(Of OrderDetail)(Me.OrderDetails)
  Dim orderDetailsCount As Integer = orderDetails.Count
  Dim counter As Integer = 0
  Do While counter < orderDetailsCount
    orderDetails(0).Delete()
      counter += 1
  Loop
  Me.Delete()
  Return (orderDetailsCount + 1)
End Function

This method remains subject to the limitations previously discussed concerning other related OrderDetails that may exist but have not yet been retrieved into the local cache; but assuming you've covered that base to your satisfaction, it will do the job. To use it, you simply call

currentOrder.DeleteGraph()

instead of:

currentOrder.Delete()

whenever you want the cascading delete. This is a simple and straightforward approach.

Provide a PersistenceManager Watcher

In this option you provide a class that watches the PersistenceManager for requests to delete Order objects, and intervenes before those deletions are consummated to delete the related child objects.**

It does the latter by means of a handler for the Order table's Row_Deleting event.

The Row_Deleting handler is set up in the class's constructor. In the implementation below, a Row_Deleted handler is also set up, and provides feedback on what occurred; but that is only for pedagogical purposes.

** The class as shown here is only watching the Order table, and only for deletions; however, clearly its scope could be enlarged to include other tables and other operations

C#:

using System;
using System.Collections.Generic;
using System.Text;
using System.Data;
using System.Windows.Forms;
using Model;
using IdeaBlade.Persistence;

namespace UI {
classPmWatcher {
    public PmWatcher(PersistenceManager pPersMgr) {
      mPersMgr = pPersMgr;
      SetWatchersForOrder();
      //Could set watchers here for other tables, as desired
    }

    privatevoid SetWatchersForOrder() {
       DataTable orderTable = mPersMgr.GetTable(typeof(Order));
       orderTable.RowDeleting +=
         newDataRowChangeEventHandler(OrderTable_RowDeleting);
orderTable.RowDeleted +=new DataRowChangeEventHandler(OrderTable_RowDeleted);
    }

#region Row_Deleting Handler
///

/// Delete related OrderDetail rows before consummating the delete of the targetted Order.
///

    void OrderTable_RowDeleting(object sender, DataRowChangeEventArgs e) {
      DeleteRelatedObjects(e);
    }

    privatestaticvoid DeleteRelatedObjects(DataRowChangeEventArgs e) {
      Order currentOrder = (Order)e.Row;
      EntityList<OrderDetail> orderDetails = new
EntityList<OrderDetail>(currentOrder.OrderDetails);
      int orderDetailsCount = currentOrder.OrderDetails.Count;
      for (int counter = 0; counter < orderDetailsCount; counter++) {
        orderDetails[0].Delete();
      }
    }
    #endregion

#region Row_Deleted Handler
///

/// Report number of items in PM that are marked for deletion.
///

    ///
    /// This handler's action is intended simply for pedagogical purposes, to provide
    ///feedback that the RowDeleting handler actually worked.
    ///
    privatevoid OrderTable_RowDeleted(object sender, DataRowChangeEventArgs e) {
      DisplayCountOfRecordsDeleted();
    }

    privatevoid DisplayCountOfRecordsDeleted() {
      EntityList<Entity> deletedEntities = new
EntityList<Entity>(mPersMgr.GetEntities(DataRowState.Deleted));
      String msg = String.Format("Here in RowDeleted handler. " +
        "There are now {0} records marked for deletion in the PM.",
deletedEntities.Count.ToString());
      MessageBox.Show(msg);
    }
    #endregion

    #region Private Fields
    PersistenceManager mPersMgr;
    #endregion
}
}

VB.NET:

Imports System
Imports System.Collections.Generic
Imports System.Text
Imports System.Data
Imports System.Windows.Forms
Imports Model
Imports IdeaBlade.Persistence

Namespace UI
Friend Class PmWatcher
   Public Sub New(ByVal pPersMgr As PersistenceManager)
     mPersMgr = pPersMgr
   SetWatchersForOrder()
   'Could set watchers here for other tables, as desired
   End Sub

   Private Sub SetWatchersForOrder()
      Dim orderTable As DataTable = mPersMgr.GetTable(GetType(Order))
      AddHandler orderTable.RowDeleting, AddressOf OrderTable_RowDeleting
      AddHandler orderTable.RowDeleted, AddressOf OrderTable_RowDeleted
   End Sub

#Region "Row_Deleting Handler"
'''

''' Delete related OrderDetail rows before consummating the delete of the targetted Order.
'''

   Private Sub OrderTable_RowDeleting(ByVal sender As Object, ByVal e As DataRowChangeEventArgs)
   DeleteRelatedObjects(e)
   End Sub

   Private Shared Sub DeleteRelatedObjects(ByVal e As DataRowChangeEventArgs)
   Dim currentOrder As Order = CType(e.Row, Order)
   Dim orderDetails As EntityList(Of OrderDetail) = New EntityList(Of
OrderDetail)(currentOrder.OrderDetails)
   Dim orderDetailsCount As Integer = currentOrder.OrderDetails.Count
   Dim counter As Integer = 0
   Do While counter < orderDetailsCount
          orderDetails(0).Delete()
          counter += 1
   Loop
   End Sub
   #End Region

#Region "Row_Deleted Handler"
'''

''' Report number of items in PM that are marked for deletion.
'''

   '''
   ''' This handler's action is intended simply for pedagogical purposes, to provide
   ''' feedback that the RowDeleting handler actually worked.
   '''
   Private Sub OrderTable_RowDeleted(ByVal sender As Object, ByVal e As DataRowChangeEventArgs)
   DisplayCountOfRecordsDeleted()
   End Sub

   Private Sub DisplayCountOfRecordsDeleted()
   Dim deletedEntities As EntityList(Of Entity) = New EntityList(Of
Entity)(mPersMgr.GetEntities(DataRowState.Deleted))
   Dim msg As String = String.Format("Here in RowDeleted handler. " & "There are now {0} records marked for deletion in the PM.", deletedEntities.Count.ToString())
   MessageBox.Show(msg)
   End Sub
   #End Region

   #Region "Private Fields"
   Private mPersMgr As PersistenceManager
   #End Region
End Class
End Namespace

This watcher must be activated somewhere, before the Order deletions can be initiated. In the attached sample app ("CascadingDeleteWithPmWatcher") we have done so in the Form_Load event handler for the application's main form. That handler calls another method that issues the following statement:

C#:

mPmWatcher = newPmWatcher(mPersMgr);

VB.NET:

mPmWatcher = NewPmWatcher(mPersMgr)

where mPmWatcher was declared elsewhere at the class level.

This approach to the problem has the advantage of leaving Delete() as the method to be used even for objects with related objects that need to be deleted. It also centralizes the exceptional delete logic, which you may consider either an advantage or a disadvantage, depending upon your perspective.

Conclusion

None of the methods illustrated for implementing cascading deletes is clearly superior to the others for all circumstances. Make your choice based on other factors in your development environment, or simply according to your personal taste. They all work!

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TechTip: Cascading Deletes : Level 200 DevForce Express View...