i want to get the defalut value of the contract_id of the rh_contract module, the field is in relation of many2one with the rh.employee form

in my customu module i want to set the value of the field to my contrat field.

i did this but something still missing 

employee_id = fields.Many2one('hr.employee', string="Employee")
def _get_default_contrat(self):
    return self.env['hr.contract'].search([], limit=1).id

contrat = fields.Many2one('hr.contract', string='Contrat', default=_get_default_contrat, domain=[])

@api.onchange('employee_id') 
def onchange_contrat_id(self): 
self.contrat = self.employee_id.contract_id