Grouping Data Across Collections

Up until now, the scope of our queries has been confined to one collection at a time. But as the data is spread across multiple collections, we may need to write queries that fetch data from more than one collection simultaneously. Suppose we need to find the product(s) bought by a specific user. To do this, we first filter the required user and retrieve their order history from the users collection. Then, using that order history, we fetch the products purchased in each order from the orders collection. 

MongoDB doesn't allow grouping across collections in a single $group stage directly, but we can combine (or join) collections first, then group them. Here are the supported methods:

• Use $lookup to join collections.

• Use $unionWith to merge similar collections.

Use $lookup to join collections

We can use $lookup to combine data from another collection before grouping. It's safe to use ...