如何在用猫鼬更新时验证数组大小

Question

我有猫鼬这个模型

const WorkOrderSchema = new Schema({
    title: {
        type: String,
        required: 'Enter a title for the order'
    }, 
    description: {
        type: String,
        required: 'Enter a description for the order'
    },
    deadline: {
        type: Date,
        required: 'Enter a deadline for the order'
    },
    workers: [{ type: Schema.Types.ObjectId, ref: "Worker" }]
});

WorkOrderSchema.path('workers').validate(function(workers) {
    if(workers.length > 5){return false}
    return true;
}, 'Too many workers');

module.exports = mongoose.model('Orders', WorkOrderSchema);

以及将工作人员分配给订单的此代码。

exports.assign_a_worker = function(req, res) {
    Order.update({"_id": req.params.orderId}, 
        {"$addToSet": {"workers": req.params.workerId}},
        {runValidators: true},
        function (err, order) {
            if (err)
                res.send(err);
            res.json(order);
        }
    );      
};

但是，它不能验证数组中工作程序的数量，并且允许我向模型中的workers数组添加无限个对象。不太确定出了什么问题。