👐 $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 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:

Atlas UI

[
  {
    $project: { title: 1, year: 1 }
  }
]

MongoDB Shell

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

• 1 means, "Show that field." Once you start an inclusion projection, you can't exclude other fields. You just keep adding the fields you want to see.
• 0 means, "Hide that field." Once you start an exclusion projection, you can't include other fields. You just keep adding the fields you don't want to see.
• The primary key _id field is shown by default.

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

Atlas UI

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

MongoDB Shell

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

👐 Show only title and cover.

Answer

Instruqt

Open the 100_aggregation_pipeline_match file in the Instruqt IDE or GitHub Codespaces. Run the Startup Code, that will import all the neccessary modules and connect to the local MongoDB cluster. Then, run the $project cell. You can then add your code to show only the title and cover fields in a new cell.

const showOnlyTitleAndCover = [
  {
    $project: { title: 1, cover: 1 }
  }
 ];

 const cursor = await books.aggregate( showOnlyTitleAndCover );

 await cursor.forEach((b) => {
    console.log(b);
 });

Atlas UI

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

MongoDB Shell

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

Excluding fields

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

Answer

Instruqt

Open the 100_aggregation_pipeline_match file in the Instruqt IDE or GitHub Codespaces. Run the Startup Code, that will import all the neccessary modules and connect to the local MongoDB cluster. Then, run the $project cell.

const removeAttributesAndCover = [
  {
    $project: {_id: 0, attributes: 0, cover: 0}
  }
 ];

 const cursor = await books.aggregate( removeAttributesAndCover );

 await cursor.forEach((b) => {
    console.log(b);
 });

Atlas UI

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

MongoDB Shell

db.books.aggregate([{$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}}])

---

Challenge 🚀

👐 1. Find books with more than 2 available copies.

Answer

JavaScript

await books.aggregate([
  { $match: {available: {$gt: 2}}}
]).toArray();

mongosh

db.books.aggregate([
  { $match: {available: {$gt: 2}}}
]);

C#

var pipeline = booksCollection.Aggregate()
  .Match(b => b.Available > 2);

var plentifulBooks = pipeline.ToList();

if (plentifulBooks != null)
{
  foreach (var book in plentifulBooks)
  {
    Console.WriteLine($"Title: {book.Title} - Available: {book.Available}");
  }
}
else
{
  Console.WriteLine("Empty Collection");
}

Python

books_two_copies = books.aggregate([
    {"$match": {"available": {"$gt": 2}}}
])

for book in books_two_copies:
    print(book)

Java

books.aggregate(
    List.of(
        Aggregates.match(gt(
            "available", 2)))
    ).forEach(document -> System.out.println(document.toJson()));

👐 2. Find books with more than 2 available copies. Return only book titles and publication year.

Answer

JavaScript

await books.aggregate([
  { $match: {available: {$gt: 2}}},
  { $project: {title: 1, year: 1, _id: 0}}
]).toArray();

mongosh

db.books.aggregate([
  { $match: {available: {$gt: 2}}},
  { $project: {title: 1, year: 1, _id: 0}}
]);

C#

var pipeline = booksCollection.Aggregate()
    .Match(b => b.Available > 2)
    .Project(b => new { b.Title, b.Year });

var plentifulBooks = pipeline.ToList();

if (plentifulBooks != null)
{
  foreach (var book in plentifulBooks)
  {
    Console.WriteLine($"Title: {book.Title} - Year: {book.Year}");
  }
}
else
{
  Console.WriteLine("Empty Collection");
}

Python

books_two_copies = books.aggregate([
    { "$match": { "available": { "$gt": 2 } } },
    { "$project": { "title": 1, "year": 1, "_id": 0 } }
])

for book in books_two_copies:
    print(f"Title: {book['title']} - Publication Year: {book['year']}")

Java

books.aggregate(
    List.of(
        Aggregates.match(gt(
            "available", 2)),
        Aggregates.project(Projections.include("title", "year")),
        Aggregates.project(Projections.exclude("_id")))
    ).forEach(document -> System.out.println(document.toJson()));