clinic/netforce_clinic/models/account_invoice.py

from netforce.model import Model, fields

class AccountInvoice(Model):
    _inherit="account.invoice"
    _fields={
        'clinic_expense_id': fields.Many2One("clinic.hd.case.expense","Expense"),
        'department_id': fields.Many2One("clinic.department","Department",search=True),
    }

AccountInvoice.register()