Whenever I create a new user, a new res.partner record is automatically generated. So, I find the create function in res.partner class in addons. But I cannot find it. And I inherited the res.partner class as follow.

class ResPartner(models.Model):
    _name = 'res.partner'
    _inherit = 'res.partner'
    _description = 'Res Partner'

    department_id = fields.Many2one('hr.department', 'Department')

    @api.model
    def create(self, vals):
        res = super(ResPartner, self).create(vals)        
        company_id = res.company_id.id
        return res

In the dictionary vals, there is no department_id, so I want to add department_id to vals. How can I 
achieve this? Thanks in advance!