I have created an API in which I am updating some parameters

If I am updating all parameters it's working fine but if I am updating separately any parameter then other parameters getting null. I don't know how to handle this condition.
I want that if I am updating one parameter it should not affect other properties.

router.put('/session/addActivity/:sessionID',function(req,res){
    Session.findOne({_id:req.params.sessionID},function(err,session){
        if(err){
            return res.send(err);
        }
        if(session){
            session.qanda.isEnable = req.body.qanda_enabled;
            session.qanda.isModerate = req.body.qanda_moderation;
            session.qanda.isCommentEnable = req.body.qanda_comments;
            session.qanda.isAnonymousResponse = req.body.qanda_anonymousResponse;
            session.poll.isEnable = req.body.poll_enabled;
            session.poll.isMultiSelect = req.body.poll_MultiSelect;
            session.poll.answerLimit = req.body.poll_answerLimit;
            session.pulse.isEnable = req.body.pulse_enabled;
            session.pulse.isAnonymousResponse = req.body.pulse_anonymousResponse;
            session.pulse.votesLimitPerUser = req.body.pulse_votesPerUser;
            session.save(function(err,session){
                if(err){
                    return res.send(err);
                }
                else{
                    res.json(session);
                }
            });
        }
    })
});

via Ikshit Mathur