SDK數(shù)據(jù)庫(kù) Aggregate·文檔拆分

Aggregate.unwind(value:string|object): Aggregate

支持端：小程序 2.7.4, 云函數(shù) 0.8.1, Web

聚合階段。使用指定的數(shù)組字段中的每個(gè)元素，對(duì)文檔進(jìn)行拆分。拆分后，文檔會(huì)從一個(gè)變?yōu)橐粋€(gè)或多個(gè)，分別對(duì)應(yīng)數(shù)組的每個(gè)元素。

參數(shù)

value: string|object

返回值

Aggregate

API 說(shuō)明

使用指定的數(shù)組字段中的每個(gè)元素，對(duì)文檔進(jìn)行拆分。拆分后，文檔會(huì)從一個(gè)變?yōu)橐粋€(gè)或多個(gè)，分別對(duì)應(yīng)數(shù)組的每個(gè)元素。

unwind 有兩種使用形式：

1. 參數(shù)是一個(gè)字段名

unwind(<字段名>)

1. 參數(shù)是一個(gè)對(duì)象

unwind({
    path: <字段名>,
    includeArrayIndex: <string>,
    preserveNullAndEmptyArrays: <boolean>
})

示例

拆分?jǐn)?shù)組

假設(shè)我們有一個(gè) products 集合，包含數(shù)據(jù)如下：

{ "_id": "1", "product": "tshirt", "size": ["S", "M", "L"] }
{ "_id": "2", "product": "pants", "size": [] }
{ "_id": "3", "product": "socks", "size": null }
{ "_id": "4", "product": "trousers", "size": ["S"] }
{ "_id": "5", "product": "sweater", "size": ["M", "L"] }

我們根據(jù) size 字段對(duì)這些文檔進(jìn)行拆分

db.collection('products')
  .aggregate()
  .unwind('$size')
  .end()

輸出如下：

{ "_id": "1", "product": "tshirt", "size": "S" }
{ "_id": "1", "product": "tshirt", "size": "M" }
{ "_id": "1", "product": "tshirt", "size": "L" }
{ "_id": "4", "product": "trousers", "size": "S" }
{ "_id": "5", "product": "sweater", "size": "M" }
{ "_id": "5", "product": "sweater", "size": "L" }

拆分后，保留原數(shù)組的索引

我們根據(jù) size 字段對(duì)文檔進(jìn)行拆分后，想要保留原數(shù)組索引在新的 index 字段中。

db.collection('products')
  .aggregate()
  .unwind({
      path: '$size',
      includeArrayIndex: 'index'
  })
  .end()

輸出如下：

{ "_id": "1", "product": "tshirt", "size": "S", "index": 0 }
{ "_id": "1", "product": "tshirt", "size": "M", "index": 1 }
{ "_id": "1", "product": "tshirt", "size": "L", "index": 2 }
{ "_id": "4", "product": "trousers", "size": "S", "index": 0 }
{ "_id": "5", "product": "sweater", "size": "M", "index": 0 }
{ "_id": "5", "product": "sweater", "size": "L", "index": 1 }

保留字段為空的文檔

注意到我們的集合中有兩行特殊的空值數(shù)據(jù)：

...
{ "_id": "2", "product": "pants", "size": [] }
{ "_id": "3", "product": "socks", "size": null }
...

如果想要在輸出中保留 size 為空數(shù)組、null，或者 size 字段不存在的文檔，可以使用 preserveNullAndEmptyArrays 參數(shù)

db.collection('products')
  .aggregate()
  .unwind({
      path: '$size',
      preserveNullAndEmptyArrays: true
  })
  .end()

輸出如下：

{ "_id": "1", "product": "tshirt", "size": "S" }
{ "_id": "1", "product": "tshirt", "size": "M" }
{ "_id": "1", "product": "tshirt", "size": "L" }
{ "_id": "2", "product": "pants", "size": null }
{ "_id": "3", "product": "socks", "size": null }
{ "_id": "4", "product": "trousers", "size": "S" }
{ "_id": "5", "product": "sweater", "size": "M" }
{ "_id": "5", "product": "sweater", "size": "L" }