contract/contract_sale_invoicing/models/sale_order.py

# Copyright (C) 2019 Open Source Integrators
# Copyright (C) 2019 Serpent Consulting Services
# License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl).
from odoo import fields, models


class SaleOrder(models.Model):
    _inherit = 'sale.order'

    contract_id = fields.Many2one('contract.contract', string='Contract')