I have value error in my classname here, what should I do?

from odoo import models,fields,api
from docutils.nodes import field

class AccountInvoices(models.Model):
_inherits = {'stock.picking':'backorder_id','account.invoice':'refund_invoice_id'}

backorder_id = fields.Many2one('stock.picking','Back Order of')
refund_invoice_id = fields.Many2one('account.invoice',string="Invoice for which this invoice is the refund")


@api.depends('backorder_id','refund_invoice_id')
def _write(self, vals):
print(self.backorder_id.min_date)
print('=============================')
print(self.refund_invoice_id.date_invoice)
return super(AccountInvoice,self)._write(vals)

I create a new classname but it doesn't accept it, what will I do to call the two models, I'm trying to fetch the field min_date from stock.picking and field date_invoice from account_invoice,

I tried to use this:

from odoo import models,fields,api
from docutils.nodes import field

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

backorder_id = fields.Many2one('stock.picking','Back Order of')
date_invoice = fields.Date("This is invoices")


@api.depends('backorder_id')
def _write(self, vals):
print(self.backorder_id.min_date)
print('=============================')
print(self.date_invoice)
return super(AccountInvoice,self)._write(vals)

But the value of backorder_id.min_date is False when the actual value is 2019-08-01