....
exports.AddSomething = (req,res,next) =>{
const newSomething = new SomeEntity({
_id:new mongoose.Types.ObjectId(), //its very own ID
somethingName:req.body.somethingName,
theForeignKey: mongoose.Types.ObjectId(req.body.theForeignKey)// if you want to pass an object ID
})
}
...