Advanced MongoDB Aggregation Pipeline: Using $lookup for Multi-Collection Joins

In MongoDB aggregation pipelines, `$lookup` is used for multi-collection association queries, similar to JOIN operations in relational databases. It requires specifying the target collection (`from`), the matching field in the current collection (`localField`), the matching field in the target collection (`foreignField`), and the field to store the result (`as`). The matching results are stored in the `as` field as an array. For example, to associate the `users` collection with the `orders` collection, using `localField: "_id"` and `foreignField: "userId"` for matching allows retrieving each user's order list. Advanced usage can combine stages like `$match` (for initial filtering) and `$unwind` (for array expansion). For instance, to count the number of orders for users over 25 years old. When using `$lookup`, note the following: ensure consistent field types, create an index on the `foreignField` of the target collection (to avoid full table scans), and un-matched data will return an empty array. `$lookup` is a core tool for multi-collection associations. Mastering its parameters and basic usage, combined with other aggregation stages, enables efficient handling of complex association scenarios.