所以标题说明了一切。似乎出于某种原因我不能访问问候功能，我真的不明白为什么。我正在跟随一位老师从课程和一切似乎是工作正常的一面，这是.js：

const mongoose = require("mongoose");
mongoose.connect("mongodb://localhost:27017/shopApp")

    .then(() => {  //check if you succesfully connect to mongodb
        console.log("Connection Open !")
    })
    .catch(err => {
        console.log("Oh No Error")
        console.log(err)
    })

const productSchema = new mongoose.Schema({
    name: {
        type: String,
        required: true,
        maxLength: 20
    },
    price: {
        type: Number,
        required: true,
        min: [0, "Price must be positive !"]
    },
    onSale: {
        type: Boolean,
        default: false
    },
    categories: [String],
    qty: {
        online: {
            type: Number,
            default: 0
        },
        inStore: {
            type: Number,
            default: 0
        }
    },
    size: {
        type: String,
        enum: ["S", "M", "L"]//these are the only string that'll be accepted 
    }
})

const Product = mongoose.model("Product", productSchema)

productSchema.methods.greet = function () {
    console.log("hello ! ")
    console.log(`- from ${this.name}`)
}

const findProduct = async () => {
    const foundProduct = await Product.findOne({ name: "Tire pump" });
    foundProduct.greet();
}
findProduct()

我知道问题是从const产品和以下，但找不到任何错误
下面是当我用node加载product.js时得到的响应：
未捕获的类型错误：foundProduct.greet不是函数