MongoDB has introduced a new feature called `$lookup`, which is an aggregation pipeline stage that allows users to insert data from a different collection into their pipeline, effectively performing a left outer join. This feature is only available in MongoDB Enterprise Server and was demoed at MongoDB World in June 2015. The decision to include `$lookup` in the Enterprise Server version was made due to the limitations of the relational database model, which can impede performance and scaling. Instead, MongoDB adopted a document-based data model that allows for more performant and scalable applications. While joins are still useful for reporting and business intelligence purposes, the full integration with BI tools is only available as part of the Enterprise Advanced subscription, requiring significant testing to ensure smooth interoperation. The `$lookup` feature can be tried out in the 3.1.8 Enterprise Server build.