Problem with a array of objects (or subdoc)

[alem...@gmail.com]

Hello everyone, I encounter a problem that prevents me from updating a field which is an array of objects.

Here's the schema:

var BSchema = new mongoose.Schema({
       owner: {type: mongoose.Schema.Types.ObjectId, ref: 'CollectionA'},
       content: [
               {
                       item: {type: mongoose.Schema.Types.ObjectId, required: true, ref: 'CollectionC'},
                       amount: {type: Number, required: true}
               }
       ]
});

This is the code used to update the content:

router.route('/')
        .put((req, res) => {
                //content being PUT here is only ids, need to translate back to object ref first
                var newcontent = req.body.content;
                newcontent.forEach((e, i, a) => a[i].item = new mongoose.Schema.Types.ObjectId(e.item));
                BModel.findByIdAndUpdate(req.session.cart._id, {content: newcontent}, (e, c) => {
                        ...
                }
        }

Whatever the update parameter is (tried { $set: { content: newcontent} }, { $push: { content: newcontent} }), all give me this same error:

{
    [CastError: Cast to embedded failed for value "[object Object]" at path "content"]
    message: 'Cast to embedded failed for value "[object Object]" at path "content"',
    name: 'CastError',
    kind: 'embedded',
    value: {
        item: ObjectId {
            path: '5647142b8706599779e15368',
            instance: 'ObjectID',
            validators: [],
            setters: [],
            getters: [],
            options: undefined,
            _index: null
        },
        amount: 3
    },
    path: 'content',
    reason: undefined
}

As you can see the value should conform to the embedded doc definition in the schema.
This is so cryptic that I can't understand how this happened.

Whatever document I can find is about how to update array by inserting elements, what I want is replace that array with a new one.
What is the proper way to do this?

[alem...@gmail.com]

Removing the ObjectId casting fixes the problem. I wonder why passing a ObjectId object is wrong.