Aggregate.lookup

不支持在小程序端进行

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。

API 定义

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:

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

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 ordersbooks 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 ordersbook 字段和 bookstitle 字段,并将 books 匹配结果直接 mergeorders 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 ordersbooks 集合,要求两个条件:

  1. ordersbook 字段与 bookstitle 字段相等
  2. ordersquantity 字段大于或等于 booksstock 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
      }
    ]
  }
]