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👐 $lookup (JOINs in MongoDB)

$lookup in MongoDB serves a similar purpose to JOINs in SQL. It enables combining data from multiple collections by performing an equality match between fields.

🔹 Understanding $lookup​

The $lookup stage is used to fetch related documents from another collection, similar to INNER JOIN or LEFT JOIN in SQL.

Syntax​

{
  $lookup: {
    from: "<foreign_collection>",  // Collection to join
    localField: "<local_field>",   // Field in current collection
    foreignField: "<foreign_field>", // Field in foreign collection
    as: "<output_array>"             // Output field
  }
}
  • from: The target collection for joining.
  • localField: The field in the current collection.
  • foreignField: The field in the target collection.
  • as: The output field containing the joined data as an array.

🔹 Joining 2 collections​

👐 Example 1: Joining Authors collection with Books​

🔍 Get book details along with their authors

db.authors.aggregate([
  {
    $lookup: {
      from: "books",
      localField: "books",
      foreignField: "_id",
      as: "booksWritten",
    },
  },
]);

Equivalent SQL query​

SELECT author.* , books_written.*
FROM authors
LEFT OUTER JOIN author_books ON authors.id = author_books.author_id
LEFT OUTER JOIN books as books_written ON author_books.book_id = books_written._id;
info

The result in MongoDB will have an array (authorDetails) instead of flat columns.

🔹 Handling unwinding ($unwind)​

Since $lookup produces an array, we can flatten it using $unwind.

Example 2: Get only book titles and single author name​

db.authors.aggregate([
  {
    $lookup: {
      from: "books",
      localField: "books",
      foreignField: "_id",
      as: "booksWritten",
    },
  },
  { $unwind: "$booksWritten" },
  { $project: { name: 1, "booksWritten.title": 1, _id: 0 } },
]);
info

The $lookup operation creates an array within each book document. Using $unwind then flattens this array, resulting in a separate document for every single book-author pair.

👐 Challenge​

👐 1. Fetch books with their associated reviews​

Answer
db.books.aggregate([
  {
    $lookup:
      {
        from: "reviews",
        localField: "_id",
        foreignField: "bookId",
        as: "reviews"
      }
  }
]);