I'm trying to get 'title' automatically when customer will select. can anyone help me?

class res_partner(osv.osv):

    _name = 'project.task'

    _inherit = 'project.task'

    _columns = {

            'partner_id': fields.many2one('res.partner', string='Employees', groups='base.group_user'),

            'title':fields.char('Tte'),

    }

    def onchange_partner_id(self, cr, uid, ids, partner_id, context=None):

        if partner_id:

                partner_id = self.pool.get('res.partner').browse(cr, uid, title, context=context)

                return {'value': {'title': partner_id.title,}}

        return {'value':{}}

xml::

<field name="partner_id" on_change="onchange_partner_id(title,partner_id)"/>

<field name="title" on_change="onchange_partner_id(title,partner_id)"/>