一形庭、模型關(guān)聯(lián)

1坤邪、一對(duì)多/多對(duì)多

在一中關(guān)聯(lián)多中的字段灸撰，type為mongoose.Schema.Types.ObjectId,并關(guān)聯(lián)關(guān)聯(lián)模型的名稱(chēng)挪捕。

const categorySchema = new mongoose.Schema({
  name: {type: String}
})

const tagSchema = new mongoose.Schema({
  name: {type: String}
})

const articleSchema = new mongoose.Schema({
  title: {type: String},
  content: {type: String},
  /* 一對(duì)多字段為一個(gè)對(duì)象 */
  category: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'Category'   // 關(guān)聯(lián)的模型
  },
  /* 多對(duì)多字段為一個(gè)數(shù)組 */
  tags: [{
    type: mongoose.Schema.Types.ObjectId,
    ref: 'Tag' 
  }]
})

2粗梭、關(guān)聯(lián)模型的查詢(xún)

(1)一對(duì)多查詢(xún)

從關(guān)聯(lián)對(duì)象中查詢(xún)

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

結(jié)果為一個(gè)數(shù)組,數(shù)組中的對(duì)象含有category對(duì)象

[ { tags: [ 5ced005cf0b6b50fe429ffdb, 5ced005df0b6b50fe429ffdc ],
    _id: 5cecff6c9d129a1520c4fa9c,
    title: 'article1',
    content: 'article1',
    __v: 1,
    category: { _id: 5ced0007037e041ec0560c1a, name: 'node', __v: 0 } },
  { tags: [ 5ced005df0b6b50fe429ffdc ],
    _id: 5cecff6d9d129a1520c4fa9d,
    title: 'article2',
    content: 'article2',
    __v: 1,
    category: { _id: 5ced0008037e041ec0560c1b, name: 'vue', __v: 0 } } ]

(2)多對(duì)多查詢(xún)

  const tag = await Article.find().populate('tags')
  console.log(tag)

結(jié)果為數(shù)組，被關(guān)聯(lián)對(duì)象字段也是一個(gè)數(shù)組级零，包含多個(gè)對(duì)象

[ { tags: [ [Object], [Object] ],
    _id: 5cecff6c9d129a1520c4fa9c,
    title: 'article1',
    content: 'article1',
    __v: 1,
    category: 5ced0007037e041ec0560c1a },
  { tags: [ [Object] ],
    _id: 5cecff6d9d129a1520c4fa9d,
    title: 'article2',
    content: 'article2',
    __v: 1,
    category: 5ced0008037e041ec0560c1b } ]

3断医、從被關(guān)聯(lián)模型中查詢(xún)關(guān)聯(lián)模型

(1)設(shè)置虛擬字段

在被關(guān)聯(lián)模型中設(shè)置虛擬字段

categorySchema.virtual('articles', {  // 定義虛擬字段
  ref: 'Article',                                       // 關(guān)聯(lián)的模型
  localField: '_id',                                // 內(nèi)鍵，Category模型的id字段
  foreignField: 'category',                     // 外鍵妄讯，關(guān)聯(lián)模型的category字段
  justOne: false                                        // 只查詢(xún)一條數(shù)據(jù)
})

const tagSchema = new mongoose.Schema({
  name: {type: String}
})
tagSchema.virtual('articles', {
  ref: 'Article',
  localField: '_id',
  foreignField: 'tags',
  justOne: false
})

(2)查詢(xún)

const article = await Tag.find().populate('articles')
console.log(article)

結(jié)果表面上看起來(lái)沒(méi)有其他字段的信息孩锡，但虛擬字段的內(nèi)容已經(jīng)包括在內(nèi)了;

[ { _id: 5ced005cf0b6b50fe429ffdb, name: 'tag1', __v: 0 },
  { _id: 5ced005df0b6b50fe429ffdc, name: 'tag2', __v: 0 } ]

繼續(xù)在結(jié)果的數(shù)組中選擇一個(gè)對(duì)象，并查詢(xún)articles字段亥贸；

const article = await Tag.find().populate('articles')
console.log(article[0].articles)

結(jié)果就會(huì)為tag為tag1的所有文章。

[ { tags: [ 5ced005cf0b6b50fe429ffdb, 5ced005df0b6b50fe429ffdc ],
    _id: 5cecff6c9d129a1520c4fa9c,
    title: 'article1',
    content: 'article1',
    __v: 1,
    category: 5ced0007037e041ec0560c1a } ]