mongoose-mpath module is creating the document but not assigned to it's parent

const {category, parent} = newCategory;
        const newCat = new Category({
            _id: new mongoose.Types.ObjectId(),
            name: category,
            parent: parent
        }).save((err, res) => {
            if(err) { console.log(err)}
            else{
                console.log("res in create category:", res);
                // io.to(user.room).emit('create-category-to-list', {res});
            }
        })

In log

   res in create category: {    
    codewords: [],  
    children: [],  
    _id: 604916cf866a154e284b2e29, 
    name: 'nameofasdasdasdcat',   
    parent: 60490c9ce00e8b7a38cf4752,
    path: '60490c9ce00e8b7a38cf4752#604916cf866a154e284b2e29',  
    _v:0
}

Here the document is created but not assigned to its parent why this is happening.

Here you can see parent children array is empty

root: {     
    children: [],  
    _id: 60490c9ce00e8b7a38cf4752,    
    name: 'root',         
    path: '60490c9ce00e8b7a38cf4752',   
    _v:0
}

Here i am using mongoose pulgin(mongoose-mpath) for create a tree form data.

For mongoose-mpath https://www.npmjs.com/package/mongoose-mpath

In getChildrenTree function need to pass options as lean:false

const parent = await Folder.findOne({ _id: user.rootId });
const tree = await parent.getChildrenTree(
   { 
     populate: 'codewords', 
     options: { lean: false } //you need to pass options as lean: false
   });
console.log({ tree });

In log

{
  tree: [
    {
      codewords: [],
      children: [],
      _id: 604c6c5bc8a2e03ee43c4c7b,
      name: 'Category-name-2',
      parent: 604c690b87924705a401f9ce,
      path: '604c690b87924705a401f9ce#604c6c5bc8a2e03ee43c4c7b',
      __v: 0
    },
    {
      codewords: [],
      children: [],
      _id: 604c6ce2c8a2e03ee43c4c7d,
      name: 'Category-name-2',
      parent: 604c690b87924705a401f9ce,
      path: '604c690b87924705a401f9ce#604c6ce2c8a2e03ee43c4c7d',
      __v: 0
    },
    {
      codewords: [],
      children: [Array],
      _id: 604c6d21c8a2e03ee43c4c7e,
      name: 'Category-name-3',
      parent: 604c690b87924705a401f9ce,
      path: '604c690b87924705a401f9ce#604c6d21c8a2e03ee43c4c7e',
      __v: 0
    }
  ]
}

for your better understanding you can check mongoose-mpath github issue section ==> https://github.com/vikpe/mongoose-mpath/issues/10