In an Jinja script, I want to access the previous document. I.e. from a Sales invoice, I want to access the associated Sales order. From a Sales order, I want to access the associated Quotation. From a Quotation, I want to access the associated Opportunity. The latter I can do with quotation_doc.opportunity. For the former two, the corresponding code does not work: neither sales_order_doc.quotation nor sales_invoice_doc.sales_order do work.
In the source code of ERPNext, I have found doc.prevdoc_docname, but this does not work in Jinja either.