contract/contract_sale/models/sale_order.py

# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
from odoo import api, fields, models


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

    contract_id = fields.Many2one(
        comodel_name="contract.contract",
        string="Contract",
    )