I had a many2one field which shows value while i change partner_id.But the problem is that those partner who has no the value also shows the previous value assigned in many2one field.Those partner who has no the value must show the many2one field empty .How can i resolve it.. 

Here is my code:

obj = self.pool.get('bank.internal.transfer')
id1 = obj.search(cr, uid, [('partner_id','=',name)])
for i in obj.browse(cr, uid, id1, context=context):
    bank_id_= i.id
    if bank_id_:
        res['bank_id']=bank_id_
    else:
        res['bank_id']=False