One-to-one join

Sometimes, you need to join data between two collections, where one document in the first collection maps to one, and only one, document in the second collection. This section provides an example of performing this one-to-one join using the $lookup aggregation stage in MongoDB.

Note

For this example, you require MongoDB version 4.4 or above. This is because you will use the $first array operator introduced in version 4.4.

Scenario

You want to generate a report to list all shop purchases for 2020, showing the product's name and category for each order, rather than the product's ID. To achieve this, you need to take the customer orders collection and join each order record to the corresponding product record in the products collection. There is a many-to-one relationship between both collections, resulting in a one-to-one join when matching an order to a product. The join will use a single field comparison between both sides, based on the product...