How do you get last insert ID after changing primary key?

I want to "insert or update if exist" a table where the column is 'mac' is to be unique and not the primary key.
So I used:
            //change primary key to mac
            $this->Office->Customer->primaryKey = 'mac';
            $customer['Customer']['mac'] = $mac;

            if($this->Office->Customer->save($customer)){
                 //get the Customer ID that was inserted or updated
                 $office['Office']['customer_id'] = $this->Office->Customer->getInsertID();
                   ....
                   ....
            }

Because I change primaryKey, getInsertID returns NULL.  Setting the column to unique causes a duplicate error.

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