👐 SELECT → projection

In SQL, the SELECT statement allows us to specify which columns to retrieve from a table. Similarly, in MongoDB, we use projection in the .find() method to control which fields to include (or exclude) in query results.

Syntax

db.collection.find({ <query> }, { <projection> });

Projection basics

• By default, MongoDB returns all fields in a document.
• Use projection to include (1) or exclude (0) specific fields.
• The _id field is always included unless explicitly excluded. 1

Example 1: Retrieve only title and authors fields

db.books.find({}, { title: 1, authors: 1, _id: 0 });

Equivalent SQL query:

SELECT title, authors FROM books;

Here:

• {} means "match all documents."
• { title: 1, authors: 1, _id: 0 } specifies that only title and authors should be returned, and _id should be excluded.

---

Example 2: Exclude the reviews field

db.books.find({}, { reviews: 0 });

Equivalent SQL query:

SELECT title, authors, genres, totalInventory, available FROM books;

Here:

• We are removing reviews, but all other fields will still appear.

---

Example 3: Using projection along with a query

db.books.find({ "genres": "Science" }, { title: 1, totalInventory: 1, _id: 0 });

Equivalent SQL query:

SELECT title, totalInventory FROM books WHERE genres='Science';

Here:

• We first filter books that belong to the "Science" genre.
• We then select only the title and totalInventory fields.

---

Challenge

👐 1. Retrieve only the title field for all books.

Answer

JavaScript

const cursor = await books.find({}).project( {title: 1, _id: 0} ).limit(10);

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

mongosh

db.books.find({}, {title: 1, _id: 0});

C#

var projection = Builders<Book>.Projection.Include(b => b.Title).Exclude(b => b.Id);

var booksWithOnlyTitle = booksCollection.Find(b => true).Project<Book>(projection).Limit(50).ToList();

if (booksWithOnlyTitle != null)
{
  foreach (var book in booksWithOnlyTitle)
  {
    Console.WriteLine(book.ToJson()); // Shows the entire BSON document as JSON to show that some fields are null because they are not returned due to the projection
  }
}
else
{
  Console.WriteLine("Empty Collection");
}

👐 2. Retrieve all fields except _id and authors for books in the "History" genre.

Answer

JavaScript

const cursor = await books.find({ "genres": "History" }).project( { _id: 0, authors: 0 } ).limit(10);

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

mongosh

db.books.find({ "genres": "History" }, { _id: 0, authors: 0 });

C#

  var historyGenre = Builders<Book>.Filter.AnyEq(b => b.Genres, "History");
  var projection = Builders<Book>.Projection.Exclude(b => b.Id).Exclude(b => b.Authors);

  List<Book> sortedBooks = booksCollection.Find(historyGenre)
    .Project<Book>(projection).Limit(50).ToList();

  if (sortedBooks != null)
  {
      foreach (var book in sortedBooks)
      {
        Console.WriteLine(book.ToJson()); // Shows the entire BSON document as JSON to show that some fields are null because they are not returned due to the projection
      }
  }
  else
  {
    Console.WriteLine("Empty Collection");
  }