Mongoose not saving nested documents

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

5.12.14

Node.js version

v16.13.1

MongoDB server version

5.0.11

Description

When modifying a field on a populated document, changes should be saved, when saving the parent.

This is not occ, instead I need to save the document myself (see Steps to Reproduce)

Steps to Reproduce

Schemas

const boardSchema = new Schema({
    name: {
        type: String,
        required: true
    },
    tasks: [
        {
            type: Schema.Types.ObjectId,
            ref: 'Task'
        }
    ]
})

const taskSchema = new mongoose.Schema({
    name: {
        type: String
    },
    description: String
})

route to change name

router.patch('/:boardId/:taskId', async (req, res) => {
    const { boardId, taskId } = req.params
    const { value } = req.body
    const board = await Board.findById(boardId).populate('tasks')

    board.tasks.find(x => x._id.toString() === taskId).name = value

    //await board.tasks[0].save() // <--------- Saving changes to task[0] // THIS SHOULD NOT BE REQUIRED
    await board.save() // <-------------------- Not saving changes to nested tasks (e.g. the name)
    res.json(board) // <----------------------- Showing correct change
    res.send('OK')
})

Expected Behavior

https://mongoosejs.com/docs/subdocs.html#subdocuments

Saving the tasks individually should not be required. They should be saved when the parent is being saved

[vkarpov15]

@jamesbruice when you populate a ref, you get a populated document, which is a distinct top-level document, rather than a subdocument embedded in your document. The difference is that an embedded subdocument is stored in MongoDB as a subdocument, rather than as an ObjectId that refers to another document.

[IslandRhythms]

A ref indicates what Model populate should look at when querying the indicated path. A subdocument is described in the image you attached to this issue.