>>> for request in pr:
    ...     sorted_quotations = sorted(request.quotation_lines,
    ...         key=lambda s: (getattr(s, 'unit_price') is None,
    ...         getattr(s, 'unit_price')))
    ...     for idx, value in enumerate(request.quotation_lines):
    ...         _ = [request.quotation_lines.pop(idx)]
    ...     _ = [request.quotation_lines.append(q) for q in sorted_quotations]
    ...     request.quotation_lines.set_sequence()
    ...     request.save()
    >>> for request in pr:
    ...     request.quotation_lines[0].unit_price
    >>> PurchaseRequest = Model.get('purchase.request')
    >>> prequest = PurchaseRequest.find([('state', '=', 'answered')])
    >>> len(prequest)
    2