Mongoose 按查找字段分组

Posted 2023-03-11

【中文标题】Mongoose 按查找字段分组

【英文标题】：Mongoose group by lookup field

【发布时间】：2018-02-03 12:46:41

【问题描述】：

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”

【参考方案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(...);

【讨论】：

非常感谢@mikey。但是这段代码有一个小问题。问题是$first。这个 $first 显示了具有 category.id 的第一个组。但是，当我有两个 subcategories 用于同一类别时，它就不起作用了。我尝试改用 $sum 但效果不佳。

@brunnogrillo 也许尝试翻转第 4 阶段和第 5 阶段。基本上，在第 3 阶段之后，我们将获得所需的所有必要信息。然后在第4阶段，取出相关字段，包括获取每个类别+子类别文档的数量。然后是第 5 阶段，按类别对数量求和。

实际上，经过您的编辑，它可以工作。谢谢@mickey，这很有帮助