MongooseJS：.populate() 不返回父字段 model

Question

目前有 2 個模型產品和書籍； Book 繼承自 Product，如下所示：

const ProductSchema = new mongoose.Schema(
  {
    name: {...},
    description: {...},
    images: [{... }],
    inventory: { ... },
    department: { ... }, 
    ....
  },
  {
    timestamps: true,
    toJSON: { virtuals: true },
    toObject: { virtuals: true },
    discriminatorKey: "kind",
  }
)
 
Model = mongoose.model("Product", productSchema)  


const BookSchema = new mongoose.Schema({
    subtitle: { ...  },
    abstract: {  ... },
    publisher: {  ... },
    authors: { ...  },
    ...

},
    {
        timestamps: true, discriminatorKey: "kind",
        toJSON: { virtuals: true },
        toObject: { virtuals: true }
    }
)

Book = Product.discriminator("Book", BookSchema) 

此外，還有一個 Cart 模式，它有一個子文檔“products”，其中包含一個引用字段“bookID”，如下所示：

const cartItem = new mongoose.Schema({
    productID: {
        type: mongoose.Types.ObjectId,
        ref: "Product",
        required: [true, "Please provide productID. "]
    },
    quantity: { ... },
    sessionID: { ... },
}) 

const cartSchema = new mongoose.Schema({
    products: [cartItem],
    active: {
        type: Boolean,
        default: true,
        hide: true,
    },
    sessionID: {
        type: mongoose.Types.ObjectId,
        ref: "Session"
    }
}, {
    timestamps: true,
    toJSON: { virtuals: true },
    toObject: { virtuals: true },
})

Cart = mongoose.model("Cart", cartSchema)

我正在使用 mongoosejs v-6.8.3

問題是 Cart 實例上的 .populate() 僅返回 Book model 中的字段（不包括 Product 模型中的字段）。

newCart = new Cart({...}) 
newCart.save()
let populatedCart = await newCart.populate({ path: "products.productID", model: Product})

Answer 1

您的問題可能是 Mongoose 的鑒別器功能如何運作的結果。 Mongoose 將為子 model 構建一個新集合，並在您使用鑒別器方法生成新的 model 時將其文檔存儲在該集合中，該新 model 繼承現有的 model。填充時查詢的唯一集合在 Cart 實例上運行，它只填充 Book model 中的字段。此問題的一個潛在解決方法是在子模型的架構中顯式包含父 model 中的字段。 以這種方式，當對 Cart 實例執行填充時，將包含來自 Product model 的字段。 另一種方法是執行 fill 方法，並選擇選擇選項以包括父 model 中的所需字段。這是一個示例：

const ProductSchema = new mongoose.Schema({
  name: {...},
  description: {...},
  images: [{... }],
  inventory: { ... },
  department: { ... }, 
  ....
}, {
  timestamps: true,
  toJSON: { virtuals: true },
  toObject: { virtuals: true },
  discriminatorKey: "kind"
});

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

const BookSchema = new mongoose.Schema({
    subtitle: { ...  },
    abstract: {  ... },
    publisher: {  ... },
    authors: { ...  },
    ...
}, {
    timestamps: true,
    toJSON: { virtuals: true },
    toObject: { virtuals: true },
    discriminatorKey: "kind",
});

const Book = Model.discriminator("Book", BookSchema); 

const cartItem = new mongoose.Schema({
    productID: {
        type: mongoose.Types.ObjectId,
        ref: "Product",
        required: [true, "Please provide productID. "]
    },
    quantity: { ... },
    sessionID: { ... },
});

const cartSchema = new mongoose.Schema({
    products: [cartItem],
    active: {
        type: Boolean,
        default: true,
    },
    sessionID: {
        type: mongoose.Types.ObjectId,
        ref: "Session"
    }
}, {
    timestamps: true,
    toJSON: { virtuals: true },
    toObject: { virtuals: true },
});

const Cart = mongoose.model("Cart", cartSchema);

let newCart = new Cart({...});
await newCart.save();
let populatedCart = await Cart.findById(newCart._id).populate({ path: "products.productID", model: "Product", select: '-__v' });
console.log(populatedCart);