How to extend Project Task in the Purchase Order header?

Question

Hi everybody,
I am facing a problem I am sure is not so hard, but I cannot find my way through it.
So, my objective is to extend the Purchase Order Header with the Project Task ID.
I created the extension field first, as below:
But I just cannot find how to populate it automatically from the data in the Project Task field.
I tried a Relation in a BO extension going through the AccountingBlockAssignement, but it seems to not be functional.
I also tried to make a Reference to the extension field and created an Event After Modify like:
this.projecttaskpo = this.ExtensionForPO.content
The Project Task PO is my reference to the extension field created in the Cloud Solution.
And the ExtensionForPO is my relation to the TaskID:
import AP.Common.GDT;
import AP.Purchasing.Global as Purchasing;
import AP.ProjectManagement.Global as ProjectManagement;
import AP.IS.CodingBlock.Global as Coding;
[Extension] businessobject AP.Purchasing.Global:PurchaseOrder {
  [Relation(Coding:AccountingCodingBlockDistribution.AccountingCodingBlockAssignment.ProjectTaskKey.TaskID)]
  [Label("Project ID")] element ExtensionForPO : TaskID;
    node Item {
I am pretty sure this is not so hard, but I can't figure how to do it.
The Task ID is in the Line Item so I do not understand why it could be so hard to add it in the Purchase Order Header.
Moreover, when I try to make the path directly from the PurchaseOrder, like:
PurchaseOrder.Item.ItemAccountingCodingBlockDistribution.AccountingCodingBlockAssignment.ProjectTaskKey.TaskID
It returns to me that the association between Item and ItemAccountingCodingBlockDistribution is not a node. I know that, but it would be perfect if I could directly go from the Purchase Order BO to the Project Reference and the Project Tasks. But I cannot do that.
Any help on this would be greatly appreciated as it is quite a recurring request from customers.
I could have a misunderstanding in the process of doing it, or even in the code as I am a pure development newbie, so I m open to all propositions.
Thank you all.
Have a nice day.
Jacques-Antoine

Answer 1

Jacques-Antoine,
You can't directly translate a repeating node or element (such as an Item or a Project Task from the Accounting Coding Block of an Item) to a singular node or element.
The reason is that, though your use case may have the elements assumed to be the same for all items, this isn't necessarily the case, so ByDesign won't assume that you can use that kind of logic.
The simplest approach would be to do an On-Save at the Root node of the Purchase Order along the following lines:
this.projecttaskpo = this.Item.GetLast().ItemAccountingCodingBlockDistribution.AccountingCodingBlockAssignment.ProjectTaskKey.TaskID;
i wouldn't actually recommend this code.
You'd need validations for IsInitial, IsSet, and those kinds of functions.
i'd at least use some foreach loop to check that all the accounting coding blocks were for the same task, raise warnings if they weren't, etc.