In calendar.view_calendar_event_form I want to make partner_ids field readonly if user_id != current_user_id. but name 'current_user_id' is not defined

I try this way, but it doesn't work.

        <xpath expr="//field[@name='partner_ids']" position="attributes">
            <attribute name="attrs">{'readonly':[('user_id','not in',uid)]}</attribute>
        </xpath>

How to check if current user is the event owner (user_id)?