add object to array and refer to other collection express mongodb code example

Example 1: add items to a list in a document monoose

const {name , id} = req.body
    Category.update(
        {_id: id}, 
        {$push: {items: {"name": name}}},{new: true, upsert: true }).exec();
        res.sendStatus(200)

Example 2: how to append data to a field in mongoose model

//mongoose appending data

var objFriends = { fname:"fname",lname:"lname",surname:"surname" };
Friend.findOneAndUpdate(
   { _id: req.body.id }, 
   { $push: { friends: objFriends  } },
  function (error, success) {
        if (error) {
            console.log(error);
        } else {
            console.log(success);
        }
    });
)