Why it is false, this is the code

from import,
import
from time, date, time
from odoo.addons.base import

class AccountInvoice (models.Model)
_inherit = 'account.invoice'

backorder_id fields.Many2one = ('stock .picking' 'Back Order of' required = True )
date_invoice = fields.Date ("Invoices:")


@ api.multi
def _write (self, vals):
res = super (AccountInvoice, self) ._ write (vals)
print ("Min_Date:", self. backorder_id.min_date )
print ('=============================')
print ("Invoice:" + self.date_invoice)
return res

This code returns False, i want to get the date value of min_date but what I get is the boolean value False?

Thank u again