# [Aggregate](Aggregate).lookup(object: Object): [Aggregate](Aggregate)

> 支持端：[云函数 1.3.0](/minigame/dev/wxcloud/reference/changelog-server-sdk)

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](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: ,
  localField: ,
  foreignField: ,
  as: 
})
```

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：

```sql
SELECT *, 
FROM collection
WHERE  IN (SELECT *
                               FROM 
                               WHERE = );
```

例子：

1. 指定一个相等匹配条件
1. 对数组字段应用相等匹配
1. 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: ,
  let: { : , ..., :  },
  pipeline: [  ],
  as: 
})
```

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`](/minigame/dev/wxcloud/reference-sdk-api/database/command/Command.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`](/minigame/dev/wxcloud/reference-sdk-api/database/command/Command.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：

```sql
SELECT *, 
FROM collection
WHERE  IN (SELECT 
                               FROM 
                               WHERE  );
```

例子

1. 指定多个连接条件
1. 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：

```json
[
  {"_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：

```json
[
  {"_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：

```js
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：

```json
[
  {
    "_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：

```json
[
  {"_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：

```json
[
  {"_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：

```js
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

```json
[
  {
    "_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：

```json
[
  {"_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：

```json
[
  {"_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。

```js
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

```json
[
  {
    "_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：

```json
[
  {"_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：

```json
[
  {"_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：
1. `orders` 的 `book` 字段与 `books` 的 `title` 字段相等
1. `orders` 的 `quantity` 字段大于或等于 `books` 的 `stock` 字段

```js
const db = cloud.database()
const _ = db.command
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：

```json
[
  {
    "_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：

```json
[
  {"_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：

```json
[
  {"_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：

```js
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

```json
[
  {
    "_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
      }
    ]
  }
]
```