$lookup (joins)

$lookup performs a left outer join with another collection in the same database.

In this page:

$lookup (joins)

Syntax

javascript

{ $lookup: {
    from: "other_collection",
    localField: "local_field",
    foreignField: "foreign_field",
    as: "result_array"
} }

$lookup (joins)

$lookup takes from, localField, foreignField and as, and adds an array of matching foreign documents to each input document.

An empty array means no match. Pair it with $unwind to flatten the array.

Frequent joins may suggest embedding the data instead.

Note: Index the foreignField for fast lookups.

Example: $lookup (joins)

bash

test> db.customers.insertMany([{ _id: 1, name: "Ada" }, { _id: 2, name: "Bob" }])
{ acknowledged: true, insertedIds: { '0': 1, '1': 2 } }
test> db.orders.insertMany([{ _id: 10, customerId: 1, total: 50 }, { _id: 11, customerId: 1, total: 20 }, { _id: 12, customerId: 3, total: 5 }])
{ acknowledged: true, insertedIds: { '0': 10, '1': 11, '2': 12 } }
test> db.orders.aggregate([
...   { $lookup: { from: "customers", localField: "customerId", foreignField: "_id", as: "customer" } },
...   { $project: { total: 1, customer: "$customer.name" } }
... ])
[
  { _id: 10, total: 50, customer: [ 'Ada' ] },
  { _id: 11, total: 20, customer: [ 'Ada' ] },
  { _id: 12, total: 5, customer: [] }
]

⚠️ Run this in your own terminal or Node.js environment.

Chapter Quiz — Complete all 7 topics to unlock

0/7 topics done

Complete these topics first:

← Previous Lesson $sort/$limit/$skip Next Lesson → $unwind

$lookup (joins)

$lookup (joins)

Chapter Quiz — Complete all 7 topics to unlock

Login to run this code