What is $lookup in MongoDB?
In MongoDB, a collection is like a container where we store related data. Each collection consists of documents, and each document has its own data.
$lookup
$lookup is an operator in MongoDB that allows us to combine documents from different collections and the same database based on a condition. It is similar to the concept of performing left join in relational database.
In this answer, we will understand the function of $lookup operator in detail.
Syntax
The $lookup operator takes an input collection. It matches the documents in the input collection with the documents in the from collection, also known as the foreign collection, based on a specified localField and foreignField. It then adds the matched documents as an array field in the input collection.
db.inputCollection.aggregate([{$lookup: {from: "foreignCollection",localField: "localField",foreignField: "foreignField",as: "outputField"}}])
db: A keyword that represents our current database.inputCollection: Our input collection.aggregate: The function that allows aggregation using$lookupoperator.from: The other foreign collection we perform join with.localField: The field from the input collection.foreignField: The field from the foreign collection.as: The name of the output field created in our input collection, which contains matched documents of the foreign collection.
Note: The $lookup operator adds the documents from the foreign collection to the input collection if there is a match between the specified localField and foreignField in both collections.
Example
Let's understand the working of $lookup operator better with an example. We suppose we have a database with two collections: customers and users.
customers collection consists of the following fields:
_id
Name
2) orders collection consists of the following fields:
_id
CustomerId
Product
Now let's study these collections before and after the execution of the $lookup operator.
Note: In MongoDB, data is represented in JSON format instead of tables. The following example shows the tabular representation to help you better understand and illustrate the example.
Before lookup
Our collections before applying $lookup operator are the following:
After lookup
We apply $lookup using the following syntax:
db.customers.aggregate([{$lookup: {from: "orders",localField: "_id",foreignField: "customerId",as: "customerOrders"}}])
Our collection customers acts as our input collection.
from: We specifyordersas our foreign collection to perform join with.localField: We specify it to be the_idfield in thecustomerscollection.foreignField: We specify it to be theCustomerIdfield in theorderscollection.as: We specify the name of the new field to becustomerOrders. This field will be added in thecustomerscollection.
Our collections after applying $lookup operator will look like the following:
Run the terminal
The following commands on the terminal demonstrate how we applied $lookup operator in the above example:
Conclusion
The $lookup operator in MongoDB allows us to combine data from multiple collections based on shared fields. By specifying the input collection, local field, foreign collection, and foreign field, we can create relationships and retrieve related data.
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