如何在猫鼬中将子文档数组的特定元素标记为已修改？

Question

I have a Mongoose schema that looks like this:

let ChildSchema = new Schema({
    name:String
});

ChildSchema.pre('save', function(next){
    if(this.isNew) /*this stuff is triggered on creation properly */;
    if(this.isModified) /* I want to trigger this when the parent's name changes */;
    next();
});

let ParentSchema = new Schema({
    name: String,
    children: [ChildSchema]

});

The isNew stuff works as expected, but I want to mark the actual array elements of children as modified so the isModified stuff is triggered anytime the parent's name changes. I'm not sure how to do this.

I've tried:

ParentModel.findById(id)
    .then( (parentDocument) => {
        parentDocument.name = 'mommy'; //or whatever, as long as its different.
        if(parentDocument.isModified('name')){
            //this stuff is executed so I am detecting the name change.
            parentDocument.markModified('children');//probably works but doesn't trigger isModified on the actual child elements in the array
            for(let i=0; i < parentDocument.children.length; i++){
                parentDocument.markModified('children.'+i);//tried this as I thought this was how you path to a specific array element, but it has no effect.
            }
            parentDocument.save();//this works fine, but the child elements don't have their isModified code executed in the pre 'save' middleware
        }
    });

So my question - how do you mark specific (or all) array elements of a subdocument as modified such that their isModified property will be true? Note that my pre save middleware is getting executed fine, it but none of the items have isModified === true ;

Answer 1

Turns out the markModified method is available on the child itself (although since I'm using TypeScript I was mislead by the typing information given).

So if I do this:

ParentModel.findById(id)
    .then( (parentDocument) => {
        parentDocument.name = 'mommy'; //or whatever, as long as its different.
        if(parentDocument.isModified('name')){
            for(let child of parentDocument.children){
                child['markModified']('name');
            }
            parentDocument.save();
        }
    });

It works. Note that if I try to just mark the child itself as modified like this:

child['markModified']();

I get the error:

MongoError: cannot use the part (children of children.{ name: 'theName'}) to traverse the element <bla bla bla>

I have no idea why that is the case, but it doesn't matter, in my case having some specific field marked as modified is fine. Would be nice to know why the traverse error comes up, but my problem is fixed.