👐 WHERE → .find()

Similar to SQL's WHERE clause, the .find() method in MongoDB retrieves documents from a collection that matches a specified query.

Syntax

db.collection.find({ <query> })

• <query>: Specifies conditions to filter documents

Example: Find all books from 2020

db.books.find({ year: 2020 });

Equivalent SQL query

SELECT * FROM books WHERE year = 2020;

Filtering with find()

The find() method takes a document as its first argument. This document specifies the filter criteria. You can use a variety of expressions within the filter document:

• Comparison operators: $eq (equals), $ne (not equals), $gt (greater than), $lt (less than), $gte (greater than or equals), $lte (less than or equals), $in (in an array), $nin (not in an array)
• Logical operators: $and, $or, $not
• Element operators: $exists (check for field existence), $type (check data type)
• Evaluation operators: $regex (regular expression matching), $where (JavaScript code execution)
• Geo-spatial operators: For location-based queries
• Array operators: For querying arrays

Now, let's utilize a few MongoDB operators and create more sophisticated queries:

$or and $gt operators

Suppose we want to get all of the books that were written in 2010 or that have more than 200 pages.

  SELECT * FROM books WHERE year = 2010 OR pages > 200;

Equivalent MongoDB query:

db.books.find({
  $or: [
    { year: 2010 }, // <--- 1st condition
    { pages: { $gt: 200 } }, // <--- 2nd condition
  ],
});

$and operator

This time, instead of OR, let's query using AND:

  SELECT * FROM books WHERE year = 2010 AND pages > 200;

Equivalent MongoDB query:

db.books.find({
  $and: [
    { year: 2010 }, // <--- 1st condition
    { pages: { $gt: 200 } }, // <--- 2nd condition
  ],
});

Shorthand $and

When we are querying on two different fields and want to utilize $and, we can do so by passing a document with all the conditions, like this:

db.books.find({ year: 2010, pages: { $gt: 200 } });

As you can see, we don't have to pass an array of conditions. MongoDB implicitly considers this as $and.

👐 Challenge

Now, translate the following into a MongoDB query.

1. Find all books where totalInventory is exactly 5.

Answer

JavaScript

const cursor = await books.find({ totalInventory: 5 });

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

mongosh

db.books.find({ totalInventory: 5 });

C#

var booksWithTotalInventoryOf5 = Builders<Book>.Filter.Eq(b => b.TotalInventory, 5);

List<Book> filteredBooks = booksCollection.Find(booksWithTotalInventoryOf5).ToList<Book>();

if(filteredBooks != null)
{
    foreach(var book in filteredBooks)
    {
        Console.WriteLine($"Book Title: {book.Title} - Total Inventory: {book.TotalInventory}");
    }
}

2. Find all books with more than 300 pages.

Answer

JavaScript

const cursor = await books.find({ pages: {$gt: 300} });

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

mongosh

db.books.find({ pages: {$gt: 300} });

C#

  var booksWithMoreThan300Pages = Builders<Book>.Filter.Gt(b => b.Pages, 300);

  List<Book> filteredBooks = booksCollection.Find(booksWithMoreThan300Pages).ToList<Book>();

  if(filteredBooks != null)
  {
      foreach(var book in filteredBooks)
      {
          Console.WriteLine($"Book Title: {book.Title} - Pages: {book.Pages}");
      }
  }

3. Find books in the Science genre that are more than 300 pages long.

Answer

JavaScript

const cursor = await books.find( { "genres": "Science", pages: {$gt: 300} } );

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

mongosh

db.books.find({ "genres": "Science", pages: {$gt: 300} });

C#

var booksWithGenreScienceAndMoreThan300Pages = Builders<Book>.Filter.And(Builders<Book>.Filter.AnyEq(b => b.Genres, "Science"), Builders<Book>.Filter.Gt(b => b.Pages, 300));

var filteredBooks = booksCollection
    .Find(booksWithGenreScienceAndMoreThan300Pages)
    .ToList();

foreach (var book in filteredBooks)
{
    Console.WriteLine($"{book.Title} - Pages: {book.Pages}");
}