*SOLVED* Numbering Line Items in a grid dilemma

[ACDC]

Is there a chance of someone cracking this puzzle, I am pulling my hair out on this one

I have an Invoice with InvoiceItems(child) and want to number the InvoiceItems in a logical number sequence of 1 to 50
As I CREATE each InvoiceItem, the new InvoiceItem.LineNo must + 1 on the previously added InvoiceItem.LineNo.

I can get this working to a certain extent based on an example I found in this forum put forward by Vlad , however I have some issues handling an event of one of the items being deleted from the Invoice Item list. Then it all falls apart, re-assigning the new LineNo's is where the challenge is

These are my Rules on the InvoiceItem Object

// This sets the initial value of the InvoiceItem.LineNo so the process below behaves correctly

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If InvoiceItem IS NEW AND InvoiceItem.Invoice.CountItems>0 Then
InvoiceItem.LineNo=500 // this just has to be a higher number 
ELSE
If InvoiceItem IS NEW AND InvoiceItem.Invoice.CountItems=0 Then
InvoiceItem.LineNo=0

//This starts the process of LineNo numbering//

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If InvoiceItem IS NEW Then
CreateInvoiceItemLineNo
 

The Process 'CreateInvoiceItemLineNo'

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1. FIND InvoiceItem WHERE InvoiceItem IN Invoice.Items AND InvoiceItem.LineNo<ThisInvoiceItem.LineNo ORDER BY InvoiceItem.LineNo DESC TAKE BEST    
2. IF SEARCH_COUNT>0 Then
    ThisInvoiceItem.LineNo=ThatInvoiceItem.LineNo+ 1
    ELSE
    ThisInvoiceItem.LineNo=1 

As mentioned this works fine as long as there are no line deletions taking place. If a user decides to delete line no 4 out of 10 items each item has to be numbered again.

Any clues on how to handle re-numbering on a deletion event
or maybe something more simpler than my version along the lines of indexing the ID attribute to represent simple numbering

[Rennur]

Try using RemovedInvoiceItem prefix in a BO rule to find all instances above the removed LineNo and REDUCE found LineNos by 1:

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If InvoiceItem WAS REMOVED FROM Invoice.InvoiceItem Then 
FIND InvoiceItem WHERE InvoiceItem IN Invoice.InvocieItem AND InvoiceItem.LineNo>RemovedInvoiceItem.LineNo 
REDUCE InvoiceItem.LineNo BY 1

[ACDC]

Try using RemovedInvoiceItem prefix in a BO rule to find all instances above the removed LineNo and REDUCE found LineNos by 1:

Absolutely Brilliant!! Works like a charm Thank you, I learned something new today

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InvoiceItem.LineNo>RemovedInvoiceItem.LineNo 

the "Removed" key word, I have never seen that before.