将字段“ DocumentDate”设置为PurchFormLetter

Question

我正在使用PurchFormLetter开具发票：

 ttsbegin;

 purchTable = PurchTable::find(_deliveryHeader.PurchId, true);

 purchFormLetter = PurchFormLetter::construct(DocumentStatus::Invoice);

 purchFormLetter.update(purchTable, _deliveryHeader.InvoiceId, today(),PurchUpdate::ReceiveNow);

 ttscommit;

发票很完美，但是在VendTrans中，DocumentDate字段为空。