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👐 $project

在输出中包括特定字段​

books 集合中的文档如下所示：

{
  _id: '0395623650',
  title: 'Platero y yo / Platero and I (Spanish-English Bilingual Edition) (English and Spanish Edition)',
  authors: [
    {
      _id: '64cc2db4830ba29148db5180',
      name: 'Juan Ramón Jiménez'
    }
  ],
  pages: 64,
  year: 1994,
  synopsis: 'Selections from a classic of world literature present a picture of life in the town of Moguer, in Andalusia, Spain.\n' +
    '\n' +
    '\n' +
    '                        Presents a picture of life in the town of Moguer, in Andalusia, Spain, as seen through the eyes of a wandering poet and his faithful donkey.\n',
  cover: 'https://images.isbndb.com/covers/36/57/9780395623657.jpg',
  attributes: [
    {
      key: 'edition',
      value: 'F First Edition Thus'
    },
    {
      key: 'dimensions',
      value: 'Height: 10.499979 Inches, Length: 6.999986 Inches, Weight: 2.0502990366 Pounds, Width: 0.12499975 Inches'
    },
    {
      key: 'isbn13',
      value: '9780395623657'
    },
    {
      key: 'msrp',
      value: 1.99
    },
    {
      key: 'isbn',
      value: '0395623650'
    },
    {
      key: 'isbn10',
      value: '0395623650'
    }
  ],
  totalInventory: 1,
  available: 1,
  binding: 'Hardcover',
  language: 'en',
  longTitle: 'Platero y yo / Platero and I (Spanish-English Bilingual Edition) (English and Spanish Edition)',
  publisher: 'Clarion Books'
}

如果我们只对书名感兴趣，我们可以使用 $project 选择我们感兴趣的字段。例如，要只获取书名和年份，我们可以这样写：

[
  {
    $project: {title: 1, year: 1}
  }
]
  • 1 表示“显示该字段”
  • 0 表示“隐藏该字段”
  • 主键 _id 字段默认显示

所以我们可以排除字段，并显示除 attributes 之外的所有字段：

[
  {
    $project: {attributes: 0}
  }
]

👐 仅显示 title 和 cover。

答案
[
  {
    $project: {title: 1, cover: 1}
  }
]

排除字段​

👐 从结果中排除 cover、attributes 和 _id 字段。

答案
[
  {
    $project: {_id: 0, attributes: 0, cover: 0}
  }
]

包括和排除字段​

danger

在同一个 $project 中不能同时包括和排除字段：如果你开始包括字段，你必须一直包括字段，反之亦然。

例如，这将失败：

db.books.aggregate([{$project: {title: 1, attributes: 0}}])

MongoServerError: Invalid $project :: caused by :: Cannot do exclusion on field attributes in inclusion projection
info

这个规则的唯一例外是 _id 字段，我们可以在包括字段的 $project 中排除它。

db.books.aggregate([{$project: {title: 1, _id: 0}}])