微信小程序云开发数据库联表查找
需求分析
现在有以下集合
orders集合: 1
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5[
{"_id":4,"book":"novel 1","price":30,"quantity":2},
{"_id":5,"book":"science 1","price":20,"quantity":1},
{"_id":6}
]1
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8[
{"_id":"book1","author":"author 1","category":"novel","stock":10,"time":1564456048486,"title":"novel 1"},
{"_id":"book3","author":"author 3","category":"science","stock":30,"title":"science 1"},
{"_id":"book4","author":"author 3","category":"science","stock":40,"title":"science 2"},
{"_id":"book2","author":"author 2","category":"novel","stock":20,"title":"novel 2"},
{"_id":"book5","author":"author 4","category":"science","stock":50,"title":null},
{"_id":"book6","author":"author 5","category":"novel","stock":"60"}
]
要求进行联表查询,返回结果,匹配字段为orders的book字段和books的title字段
# 代码 1
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11const db = cloud.database()
db.collection('orders').aggregate()
.lookup({
from: 'books',
localField: 'book',
foreignField: 'title',
as: 'bookList',
})
.end()
.then(res => console.log(res))
.catch(err => console.error(err))
得到的结果为: 1
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50[
{
"_id": 4,
"book": "novel 1",
"price": 30,
"quantity": 2,
"bookList": [
{
"_id": "book1",
"title": "novel 1",
"author": "author 1",
"category": "novel",
"stock": 10
}
]
},
{
"_id": 5,
"book": "science 1",
"price": 20,
"quantity": 1,
"bookList": [
{
"_id": "book3",
"category": "science",
"title": "science 1",
"author": "author 3",
"stock": 30
}
]
},
{
"_id": 6,
"bookList": [
{
"_id": "book5",
"category": "science",
"author": "author 4",
"stock": 50,
"title": null
},
{
"_id": "book6",
"author": "author 5",
"stock": "60",
"category": "novel"
}
]
}
]
改进
上面的代码里会生成一个新的字段bookList,若要将查询结果直接并入上一级,可有:
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18var db = cloud.database()
var $ = db.command.aggregate
db.collection('orders').aggregate()
.lookup({
from: "books",
localField: "book",
foreignField: "title",
as: "bookList"
})
.replaceRoot({
newRoot: $.mergeObjects([ $.arrayElemAt(['$bookList', 0]), '$$ROOT' ])
})
.project({
bookList: 0
})
.end()
.then(res => console.log(res))
.catch(err => console.error(err))
引用
微信小程序云开发数据库联表查找