How to solve "Changes cannot be saved to the database from the event handler"?

In POOrderEntry as a POLine is created or deleted, I need to push a reference back to a custom DAC that originates the PO Line. For instance, if the PO Line is deleted, my custom DAC has the reference removed in Events.RowDeleted via:

using (PXTransactionScope ts = new PXTransactionScope())
{
    Base.Caches[typeof(MyDAC)].SetValueExt<MyDAC.pOType>(row, null);
    Base.Caches[typeof(MyDAC)].SetValueExt<MyDAC.pONbr>(row, null);
    Base.Caches[typeof(MyDAC)].SetValueExt<MyDAC.pOLineNbr>(row, null);
    Base.Caches[typeof(MyDAC)].Update(row);
    Base.Caches[typeof(MyDAC)].Persist(PXDBOperation.Update);
    ts.Complete(Base);
}

I have tried to allow the normal Persist to save the values, but it doesn't unless I call Persist (last line of my example above). The result is an error via Acuminator of "Changes cannot be saved to the database from the event handler". As I look at this, I wonder if it should be in an Long Operation instead of a Transaction Scope, but the error from Acuminator tells me I'm doing this wrong. What is the proper way to achieve my update back to "MyDAC" for each PO Line?

I have also tried initializing a graph instance for MyDAC's graph, but I get a warning about creating a PXGraph in an event handler so I can't "legally" call the graph where MyDAC is maintained.

My code compiles and functions as desired, but the error from Acuminator tells me there must be a more proper way to accomplish this.

Solution

You can add a view to the graph extension.

Then in the row deleted you will use your view.Update(row) to update your custom dac.

During the base graph persist your records will commit as long as there are no other errors found in other events.

The way you have it now commits your changes with a chance the row that was being deleted is never deleted.

Also with this change there is no need to use PXTransactionScope.

An example might look something like this...

public class POOrderEntryExtension : PXGraphExtension<POOrderEntry>
{
    public PXSelect<MyDac> MyView;

    protected virtual void _(Events.RowDeleted<POLine> e)
    {
        //get your row to update from e.Row
        var myRow = PXSelect...

        myRow.pOType = null;
        myRow.pONbr = null;
        myRow.pOLineNbr = null;

        MyView.Update(myRow);
    }
}