0$lookup helps you combine data from two collections in MongoDB, like joining tables in a spreadsheet. It lets you see related information together in one place.
db.collection.aggregate([
{
$lookup: {
from: "otherCollection",
localField: "fieldInThisCollection",
foreignField: "fieldInOtherCollection",
as: "newFieldName"
}
}
])from is the collection you want to join with.
localField is the field in your current collection.
foreignField is the matching field in the other collection.
as is the name of the new array field that will hold matched documents.
customerInfo array.db.orders.aggregate([
{
$lookup: {
from: "customers",
localField: "customerId",
foreignField: "_id",
as: "customerInfo"
}
}
])commentsList array.db.posts.aggregate([
{
$lookup: {
from: "comments",
localField: "_id",
foreignField: "postId",
as: "commentsList"
}
}
])This example creates two collections: orders and customers. Then it uses $lookup to join orders with their customer details. The result shows each order with an array customerDetails containing matching customer info.
db.orders.insertMany([
{ _id: 1, item: "Pen", customerId: 101 },
{ _id: 2, item: "Notebook", customerId: 102 }
])
db.customers.insertMany([
{ _id: 101, name: "Alice" },
{ _id: 102, name: "Bob" }
])
// Now join orders with customers
db.orders.aggregate([
{
$lookup: {
from: "customers",
localField: "customerId",
foreignField: "_id",
as: "customerDetails"
}
}
]).toArray()The $lookup stage adds an array field even if only one match exists.
If no match is found, the array will be empty.
You can use $unwind after $lookup to flatten the array if you want one document per match.
$lookup joins two collections by matching fields.
It creates a new array field with matching documents from the other collection.
Useful to combine related data stored separately for easier reading and analysis.
$lookup stage do in a MongoDB aggregation pipeline?$lookup$lookup is used to combine documents from two collections by matching fields.$lookup.$lookup joins collections = A [OK]$lookup stage in MongoDB?from, localField, foreignField, and as.orders with documents { _id: 1, productId: 101 } and products with documents { _id: 101, name: 'Pen' }, what will be the result of this aggregation on orders?
{ $lookup: { from: 'products', localField: 'productId', foreignField: '_id', as: 'productInfo' } }productId in orders is 101, which matches _id 101 in products.productInfo.$lookup stage?
{ $lookup: { from: 'users', localField: 'userId', foreignField: 'id', as: 'userDetails' } }
Assuming the users collection uses _id as the user ID field.users collection uses _id as the ID field, so foreignField must be '_id'.localField and as are valid; collection name 'users' is correct.students with { _id: 1, name: 'Alice' } and grades with multiple documents per student like { studentId: 1, score: 85 }. How do you write a $lookup to get each student with all their grades in an array called allGrades?students._id matches grades.studentId, so these are the fields to join on.from: 'grades', localField: '_id', foreignField: 'studentId', and as: 'allGrades'.Pass the test and this pattern moves unknown strong while earning +1 Grip toward your certificate.