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👐 $project

Including fields in a projection​

A document from the books collection looks like:

{
  _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'
}

If we're interested just in the titles, we can use $project to select just the fields we're interested in. As an example, to get just the book's title and year we'll write:

[
  {
    $project: {title: 1, year: 1}
  }
]
  • 1 means "show that field"
  • 0 means "hide that field"
  • the primary key _id field is shown by default

So we can exclude fields and show all fields except attributes using:

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

👐 Show only title and cover.

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

Excluding fields​

👐 Exclude the cover, attributes and _id fields from the result.

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

Including and excluding fields​

danger

You can't include and exclude fields in the same projection: if you start including, you must keep including and vice versa.

For instance, this will fail:

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

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

The only exception to this rule is the _id field, which we can exclude in an inclusion projection.

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