如何将子文档数组的特定元素标记为在mongoose中修改的元素?

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]

});

这个 isNew stuff按预期工作,但我想标记 children isModified

我试过:

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
        }
    });

所以我的问题是,如何将子文档的特定(或所有)数组元素标记为修改后的元素,使其 已修改 财产是否真实?请注意,我的预保存中间件执行得很好,但没有一项 isModified === true