1 const Categoryschema = new mongoose.Schema({
 2   name: { type: String },
 3   parent: {
 4     type: mongoose.SchemaTypes.ObjectId,
 5     ref: ‘Category‘
 6   }
 7 })
 8 
 9 const Articleschema = new mongoose.Schema({
10   title: { type: String },
11   body: { type: String },
12   categories: [{
13     type: mongoose.SchemaTypes.ObjectId,
14     ref: ‘Category‘
15   }]
16 }, {
17   timestamps: true  // 时间戳
18 })

const categories = await Category.find().populate(‘parent‘)
console.log(categories)

 1 [{
 2   __id: 5cecff6c9d129a1520c4fa9c,
 3   name: ‘公告‘,
 4   parent: {
 5     __id: 5ced0007037e041ec0560c1a,
 6     name: ‘新闻资讯‘
 7   }
 8 }, {
 9   __id: 5cecff6d9d129a1520c4fa9d,
10   name: ‘活动‘,
11   parent: {
12     __id: 5ced0008037e041ec0560c1b,
13     name: ‘新闻资讯‘
14   }
15 }]

const articles = await Article.find().populate(‘category‘)
console.log(articles)

 1 [{
 2   __id: 5cecff6c9d129a1520c4fa9c,
 3   title: ‘title1‘,
 4   body: ‘body1‘,
 5   categories: [ [Object], [Object] ]
 6 }, {
 7   __id: 5cecff6d9d129a1520c4fa9d,
 8   title: ‘title2‘,
 9   body: ‘body2‘,
10   categories: [ [Object], [Object] ]
11 }]

 1 categorySchema.virtual(‘children‘, {  // 定义虚拟字段
 2   ref: ‘Category‘,                               // 关联的模型
 3   localField: ‘_id‘,                          // 内键，Category模型的id字段
 4   foreignField: ‘parent‘,               // 外键，关联模型的category字段
 5   justOne: false                            // 只查询一条数据
 6 })
 7  8 categorySchema.virtual(‘newsList‘, {
 9   ref: ‘Article‘,
10   localField: ‘_id‘,
11   foreignField: ‘categories‘,
12   justOne: false
13 })

const article = await Category.find({ name: ‘新闻资讯‘ }).populate({
  path: ‘articles‘,
  populate: { path: ‘newslist‘ }
}).lean()
console.log(article)

 1 [{
 2   __id: 5cd3f5cf0dce1d58335b2c3f,
 3   name: ‘新闻资讯‘,
 4   children: [{
 5     __id: 5ced005cf0b6b50fe429ffdb,
 6     parent: 5cd3f5cf0dce1d58335b2c3f,
 7     name: ‘公告‘,
 8     newsList: [{
 9       __id: 5cecff6d9d129a1520c4fa9d,
10       categories: []
11     }]
12   }, {
13     ...
14   }]
15 }, {
16   ...
17 }]