I got some issues on getting id from rest.partner in Odoo. I added compute field in stock.move.line called irLot. below is code sample.

class StockMoveLine(models.Model):
_inherit = 'stock.move.line'

irLot = fields.Char(string="Internal Reference", compute='_compute_ir')


def _compute_ir(self):
    
    partner_id = self.picking_id.partner_id.id
    partner_obj = self.env['res.partner']
    obj = partner_obj.search([('id','=',partner_id)])
    
    for rec in obj:
        internal_ref = rec.ref
         
    self.irLot = internal_ref 


so my problem is when I assign to partner_id variable. there is no value is coming out even though i assigned self.picking_id.partner_id.id. there is always showing False. so I assigned id directly and it works as below.

partner_obj = self.env['res.partner'] 
obj = partner_obj.search([('id','=','112')])


How can i get id value from self.picking_id.partner_id.id?
What did I do wrong? quite noob on odoo please suggest me.