Task.findOneAndUpdate({"_id": req.body.id_project},
            {$push: {'context_task': {'task': req.body.input_task,'status': false}}},{new: true},
            function (err, doc) {
                if (err) return next(err);
                console.log(doc.context_task[doc.context_task.length-1]._id);//получаем ID новой записи 
                res.status(200).send();
            });