Reasons why the Created From field on Invoice created from Sales Order is null.

On an invoice created from a sales order the Created From field is not populated.

An invoice can be created in the UI from a sales order by clicking the Bill button or by script through the usage of nlapiTransformRecord. In both cases the Created From field maintains the link to the sales order from which the invoice was created, only if at least one of the initially present line items is still present when the record is submitted.

If after creating the invoice from the sales order and before submitting the invoice all line items are removed and new ones are added, then the Created From field is not populated with the reference to the original sales order.

For example, the following snippet creates an invoice linked through the Created From field to the sales order:

		var salesOrderId = '123456'; //sample approved sales order, not yet billed		var invoice = nlapiTransformRecord('salesorder', salesOrderId, 'invoice');		var lineItemCount = invoice.getLineItemCount('item');		for (var i=1; i < lineItemCount; i++) //looping until the last but one line item		invoice.removeLineItem('item', 1);		var invoiceid = nlapiSubmitRecord(invoice);		

However, the following code snippet creates an invoice, but the invoice's Created From field does not contain a reference to the sales order:

		var salesOrderId = '123456'; //sample approved sales order, not yet billed		var invoice = nlapiTransformRecord('salesorder', salesOrderId, 'invoice');		var lineItemCount = invoice.getLineItemCount('item');		for (var i=1; i <= lineItemCount; i++) //looping until the last line item		invoice.removeLineItem('item', 1);		//adding a new line item in order to successfully submit the record (according to the setup of the item and of the account other fields might need to be populated)		var itemId = '789';		invoice.selectNewLineItem('item');		invoice.setCurrentLineItemValue('item', 'item', itemId);		invoice.commitLineItem('item');		var invoiceid = nlapiSubmitRecord(invoice);

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