midMongoDB
How does the $lookup operator work?
Updated May 1, 2026
Short answer
$lookup performs a join between two collections in the same database.
Deep explanation
It takes 'from' (target collection), 'localField', 'foreignField', and 'as' (output array name). It's a powerful tool for relational-like queries.
Real-world example
Fetching user details alongside their order history.
Common mistakes
- Using $lookup on unindexed foreign fields, causing slow joins.
Follow-up questions
- Can you join across databases?