SDK数据库 Aggregate·联表查询
Aggregate.lookup(object: Object): Aggregate
支持端：云函数 1.3.0
聚合阶段。聚合阶段。联表查询。与同个数据库下的一个指定的集合做 left outer join(左外连接)。对该阶段的每一个输入记录，lookup 会在该记录中增加一个数组字段，该数组是被联表中满足匹配条件的记录列表。lookup 会将连接后的结果输出给下个阶段。
参数
object: Object
返回值
Aggregate
API 说明
lookup 有两种使用方式
1. 相等匹配
将输入记录的一个字段和被连接集合的一个字段进行相等匹配时，采用以下定义：
lookup({
from: <要连接的集合名>,
localField: <输入记录的要进行相等匹配的字段>,
foreignField: <被连接集合的要进行相等匹配的字段>,
as: <输出的数组字段名>
})
参数详细说明
| 参数字段 | 说明 |
|---|---|
| from | 要进行连接的另外一个集合的名字 |
| localField | 当前流水线的输入记录的字段名，该字段将被用于与 from 指定的集合的 foreignField 进行相等匹配。如果输入记录中没有该字段，则该字段的值在匹配时会被视作 null |
| foreignField | 被连接集合的字段名，该字段会被用于与 localField 进行相等匹配。如果被连接集合的记录中没有该字段，该字段的值将在匹配时被视作 null |
| as | 指定连接匹配出的记录列表要存放的字段名，这个数组包含的是匹配出的来自 from 集合的记录。如果输入记录中本来就已有该字段，则该字段会被覆写 |
这个操作等价于以下伪 SQL 操作：
SELECT *, <output array field>
FROM collection
WHERE <output array field> IN (SELECT *
FROM <collection to join>
WHERE <foreignField>= <collection.localField>);
例子：
- 指定一个相等匹配条件
- 对数组字段应用相等匹配
- 组合 mergeObjects 应用相等匹配
2. 自定义连接条件、拼接子查询
如果需要指定除相等匹配之外的连接条件，或指定多个相等匹配条件，或需要拼接被连接集合的子查询结果，那可以使用如下定义：
lookup({
from: <要连接的集合名>,
let: { <变量1>: <表达式1>, ..., <变量n>: <表达式n> },
pipeline: [ <在要连接的集合上进行的流水线操作> ],
as: <输出的数组字段名>
})
参数详细说明
| 参数字段 | 说明 |
|---|---|
| from | 要进行连接的另外一个集合的名字 |
| let | 可选。指定在 pipeline 中可以使用的变量，变量的值可以引用输入记录的字段，比如 let: { userName: '$name' } 就代表将输入记录的 name 字段作为变量 userName 的值。在 pipeline 中无法直接访问输入记录的字段，必须通过 let 定义之后才能访问，访问的方式是在 expr 操作符中用 $$变量名 的方式访问，比如 $$userName。 |
| pipeline | 指定要在被连接集合中运行的聚合操作。如果要返回整个集合，则该字段取值空数组 []。在 pipeline 中无法直接访问输入记录的字段，必须通过 let 定义之后才能访问，访问的方式是在 expr 操作符中用 $$变量名 的方式访问，比如 $$userName。 |
| as | 指定连接匹配出的记录列表要存放的字段名，这个数组包含的是匹配出的来自 from 集合的记录。如果输入记录中本来就已有该字段，则该字段会被覆写 |
该操作等价于以下伪 SQL 语句：
SELECT *, <output array field>
FROM collection
WHERE <output array field> IN (SELECT <documents as determined from the pipeline>
FROM <collection to join>
WHERE <pipeline> );
例子
- 指定多个连接条件
- 拼接被连接集合的子查询
示例
指定一个相等匹配条件
假设 orders 集合有以下记录：
[
{"_id":4,"book":"novel 1","price":30,"quantity":2},
{"_id":5,"book":"science 1","price":20,"quantity":1},
{"_id":6}
]
books 集合有以下记录：
[
{"_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"}
]
以下聚合操作可以通过一个相等匹配条件连接 orders 和 books 集合，匹配的字段是 orders 集合的 book 字段和 books 集合的 title 字段：
const 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))
结果：
[
{
"_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"
}
]
}
]
对数组字段应用相等匹配
假设 authors 集合有以下记录：
[
{"_id": 1, "name": "author 1", "intro": "Two-time best-selling sci-fiction novelist"},
{"_id": 3, "name": "author 3", "intro": "UCB assistant professor"},
{"_id": 4, "name": "author 4", "intro": "major in CS"}
]
books 集合有以下记录：
[
{"_id":"book1","authors":["author 1"],"category":"novel","stock":10,"time":1564456048486,"title":"novel 1"},
{"_id":"book3","authors":["author 3", "author 4"],"category":"science","stock":30,"title":"science 1"},
{"_id":"book4","authors":["author 3"],"category":"science","stock":40,"title":"science 2"}
]
以下操作获取作者信息及他们分别发表的书籍，使用了 lookup 操作匹配 authors 集合的 name 字段和 books 集合的 authors 数组字段：
const db = cloud.database()
db.collection('authors').aggregate()
.lookup({
from: 'books',
localField: 'name',
foreignField: 'authors',
as: 'publishedBooks',
})
.end()
.then(res => console.log(res))
.catch(err => console.error(err))
结果
[
{
"_id": 1,
"intro": "Two-time best-selling sci-fiction novelist",
"name": "author 1",
"publishedBooks": [
{
"_id": "book1",
"title": "novel 1",
"category": "novel",
"stock": 10,
"authors": [
"author 1"
]
}
]
},
{
"_id": 3,
"name": "author 3",
"intro": "UCB assistant professor",
"publishedBooks": [
{
"_id": "book3",
"category": "science",
"title": "science 1",
"stock": 30,
"authors": [
"author 3",
"author 4"
]
},
{
"_id": "book4",
"title": "science 2",
"category": "science",
"stock": 40,
"authors": [
"author 3"
]
}
]
},
{
"_id": 4,
"intro": "major in CS",
"name": "author 4",
"publishedBooks": [
{
"_id": "book3",
"category": "science",
"title": "science 1",
"stock": 30,
"authors": [
"author 3",
"author 4"
]
}
]
}
]
组合 mergeObjects 应用相等匹配
假设 orders 集合有以下记录：
[
{"_id":4,"book":"novel 1","price":30,"quantity":2},
{"_id":5,"book":"science 1","price":20,"quantity":1},
{"_id":6}
]
books 集合有以下记录：
[
{"_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"}
]
以下操作匹配 orders 的 book 字段和 books 的 title 字段，并将 books 匹配结果直接 merge 到 orders 记录中。
var 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]), '$$ROOT' ])
})
.project({
bookList: 0
})
.end()
.then(res => console.log(res))
.catch(err => console.error(err))
结果
[
{
"_id": 4,
"title": "novel 1",
"author": "author 1",
"category": "novel",
"stock": 10,
"book": "novel 1",
"price": 30,
"quantity": 2
},
{
"_id": 5,
"category": "science",
"title": "science 1",
"author": "author 3",
"stock": 30,
"book": "science 1",
"price": 20,
"quantity": 1
},
{
"_id": 6,
"category": "science",
"author": "author 4",
"stock": 50,
"title": null
}
]
指定多个连接条件
假设 orders 集合有以下记录：
[
{"_id":4,"book":"novel 1","price":300,"quantity":20},
{"_id":5,"book":"science 1","price":20,"quantity":1}
]
books 集合有以下记录：
[
{"_id":"book1","author":"author 1","category":"novel","stock":10,"time":1564456048486,"title":"novel 1"},
{"_id":"book3","author":"author 3","category":"science","stock":30,"title":"science 1"}
]
以下操作连接 orders 和 books 集合，要求两个条件：
- orders 的 book 字段与 books 的 title 字段相等
- orders 的 quantity 字段大于或等于 books 的 stock 字段
const db = cloud.database()
const $ = db.command.aggregate
db.collection('orders').aggregate()
.lookup({
from: 'books',
let: {
order_book: '$book',
order_quantity: '$quantity'
},
pipeline: $.pipeline()
.match(_.expr($.and([
$.eq(['$title', '$$order_book']),
$.gte(['$stock', '$$order_quantity'])
])))
.project({
_id: 0,
title: 1,
author: 1,
stock: 1
})
.done(),
as: 'bookList',
})
.end()
.then(res => console.log(res))
.catch(err => console.error(err))
结果：
[
{
"_id": 4,
"book": "novel 1",
"price": 300,
"quantity": 20,
"bookList": []
},
{
"_id": 5,
"book": "science 1",
"price": 20,
"quantity": 1,
"bookList": [
{
"title": "science 1",
"author": "author 3",
"stock": 30
}
]
}
]
拼接被连接集合的子查询
假设 orders 集合有以下记录：
[
{"_id":4,"book":"novel 1","price":30,"quantity":2},
{"_id":5,"book":"science 1","price":20,"quantity":1}
]
books 集合有以下记录：
[
{"_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"}
]
在每条输出记录上加上一个数组字段，该数组字段的值是对 books 集合的一个查询语句的结果：
const db = cloud.database()
const $ = db.command.aggregate
db.collection('orders').aggregate()
.lookup({
from: 'books',
let: {
order_book: '$book',
order_quantity: '$quantity'
},
pipeline: $.pipeline()
.match({
author: 'author 3'
})
.project({
_id: 0,
title: 1,
author: 1,
stock: 1
})
.done(),
as: 'bookList',
})
.end()
.then(res => console.log(res))
.catch(err => console.error(err))
结果
[
{
"_id": 4,
"book": "novel 1",
"price": 30,
"quantity": 20,
"bookList": [
{
"title": "science 1",
"author": "author 3",
"stock": 30
},
{
"title": "science 2",
"author": "author 3",
"stock": 40
}
]
},
{
"_id": 5,
"book": "science 1",
"price": 20,
"quantity": 1,
"bookList": [
{
"title": "science 1",
"author": "author 3",
"stock": 30
},
{
"title": "science 2",
"author": "author 3",
"stock": 40
}
]
}
]