Question

我有一个包含这样的对象的集合：

{
  "_id": ObjectId("52ed12c144aecc4bf004d0b6"),
  "title": "myBook",
  "summary": "This is a book summary",
  "covers": [
    "http://url.com/cover1.jpg",
    "http://url.com/cover2.jpg",
  ],
  "urls": [
    "http://url.com/myBook",
    "http://url.com/myBook/option2"
  ],
  "genres": [
    "comedy",
    "romantic"
  ],
  "created_at": ISODate("2012-12-17T23:00:00Z"),
  "modify_at": ISODate("2014-02-01T15:41:48.149Z")
}

但我只想从封面和网址属性中获取第一个值，我的输出json应该是这样的：

{
  "_id": ObjectId("52ed12c144aecc4bf004d0b6"),
  "title": "myBook",
  "summary": "This is a book summary",
  "covers": "http://url.com/cover1.jpg",
  "urls": "http://url.com/myBook"
  "genres": [
    "comedy",
    "romantic"
  ],
  "created_at": ISODate("2012-12-17T23:00:00Z"),
  "modify_at": ISODate("2014-02-01T15:41:48.149Z")
}

我想我必须使用map_reduce，但我不确定我应该如何查询，我试过了：

db.books.mapReduce( function(){ emit(this._id, this.covers); }, function(key, values){ return values[0]; }, { out: "map_reduce_covers" } )

但这似乎不起作用。

Answer 1

您可以使用$slice投影运算符来控制输出中包含数组的哪些元素。

db.books.findOne({}, {covers: {$slice: 1}, urls: {$slice: 1}})

输出：

{
  "_id": ObjectId("52ed12c144aecc4bf004d0b6"),
  "title": "myBook",
  "summary": "This is a book summary",
  "covers": [
    "http://url.com/cover1.jpg"
  ],
  "urls": [
    "http://url.com/myBook"
  ],
  "genres": [
    "comedy",
    "romantic"
  ],
  "created_at": ISODate("2012-12-17T17:00:00-0600"),
  "modify_at": ISODate("2014-02-01T09:41:48.149-0600")
}

covers和urls仍然是数组，如果您想要更改它，它会变得更复杂。

从mongo集合中的嵌套数组元素中获取特定值

1 个答案: