$lookup for joining collections in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $lookup stage do in a MongoDB aggregation pipeline?

It performs a left outer join to another collection in the same database, combining documents from both collections based on a matching field.

beginner

Which fields are required in a basic $lookup stage?

You need from (the collection to join), localField (field from input documents), foreignField (field from the from collection), and as (name of the new array field to add).

beginner

How does $lookup handle documents with no matching join field in the foreign collection?

It still includes the original document but adds an empty array in the as field, showing no matches were found.

intermediate

Can $lookup join collections on fields with different names?

Yes, by specifying localField and foreignField with the respective field names, even if they differ.

beginner

What is the output type of the joined data added by $lookup?

The joined data is added as an array field to each input document, containing all matching documents from the foreign collection.

What does the as field specify in a $lookup stage?

A The name of the new array field to store joined documents

B The foreign collection name

C The local field to match

D The condition to filter documents

The as field defines the name of the array field added to each document to hold the joined documents.

If no matching documents are found in the foreign collection, what does $lookup add?

A A null value

B An empty array in the <code>as</code> field

C No field is added

D An error is thrown

MongoDB adds an empty array in the as field to show no matches were found.

Which of these is NOT a required field in a basic $lookup?

A from

B localField

C pipeline

D foreignField

The pipeline field is optional and used for advanced joins; basic $lookup requires from, localField, and foreignField.

What type of join does $lookup perform?

A Inner join

B Right outer join

C Full outer join

D Left outer join

$lookup performs a left outer join, including all documents from the input collection.

Can $lookup join collections from different databases?

A No, only within the same database

B Yes, always

C Yes, but only with special permissions

D Only if collections have the same name

$lookup can only join collections within the same database.

Explain how the $lookup stage works in MongoDB aggregation pipelines.

Think about how two tables join in SQL but in MongoDB's document style.

Describe a scenario where you would use $lookup and what the result looks like.

Imagine combining customer info with their purchases.