Mongoose 按查找字段分组

Question

我是 NodeJS 和 Mongoose 的新手，我正在尝试做一些高级查询。

我有三个文档（类别、子类别和产品）

Category.js

var mongoose = require('mongoose');
var CategorySchema = new mongoose.Schema({
    name: { type: String, required: true }
});

mongoose.model('Category', CategorySchema);
module.exports = mongoose.model('Category');

Subcategory.js

var mongoose = require('mongoose');
var SubcategorySchema = new mongoose.Schema({
    name: { type: String, required: true },
    image: { type: Buffer, contentType: String },
    idcategory: { type: mongoose.Schema.Types.ObjectId, ref: 'Category' }
});

mongoose.model('Subcategory', SubcategorySchema);
module.exports = mongoose.model('Subcategory');

Product.js

var mongoose = require('mongoose');
var ProductSchema = new mongoose.Schema({
    name: { type: String, required: true },
    idsubcategory: { type: mongoose.Schema.Types.ObjectId, ref: 'Subcategory', required: true },
    price: { type: Number, required: true }
});

mongoose.model('Product', ProductSchema);
module.exports = mongoose.model('Product');

因此，产品有一个子类别，子类别具有一个类别。 由于我有这种结构，我想按类别计算我的产品。例如

[
  { "categoryName": "cat1": quantity: 10 },
  { "categoryName": "cat2": quantity: 5 },
  { "categoryName": "cat3": quantity: 2 }
]

我已经尝试了 $lookup 与 $group 没有成功，因为我根本不了解聚合。

猫鼬：“4.11.6”

Answer 1

试试这个

CategoryModel.aggregate([
    // stage 1: join subcategories
    {
        $lookup: {
            from: 'subcategories',      // collection to join
            localField: '_id',          // field from categories collection
            foreignField: 'idcategory', // field from subcategories collection
            as: 'subcategory'           
        }
    },
    // at this point, each document will have an additional subcategory array with all the "joined" documents from subcategories collection
    // stage 2: we need to "unwind" the subcategory array in order to join the products
    // when you unwind an array, it's like making each element in the array have its own document
    {
        $unwind: '$subcategory'
    },
    // stage 3: join products
    {
        $lookup: {
            from: 'products',               // collection to join
            localField: 'subcategory._id',  // field from our updated collection
            foreignField: 'idsubcategory',  // field from products collection
            as: 'products'
        }
    },

    // EDIT

    // stage 4: strip out all fields except _id, category name, and quantity (number of products)
    {
        $project: {
            name: true, 
            quantity: { $size: '$products' }
        }
    },
    // stage 5: group by category ID
    {
        $group: {
            _id: '$_id',                        // group by category ID
            categoryName: { $first: '$name' },  // get category name
            quantity: { $sum: '$quantity' },    // sum the quantities
        }
    }
]).exec(...);