&gt; In Mongo, I&#x27;m pretty sure you need to first lookup engineering, then lookup the employees in engineering. That could be O(# employees in engineering) queries rather than 1.<p>The problem with that summary boils down to bad architecture. The point of document storage is storage with purpose; the intent being to make querying EASIER. This could easily be structured to be a single query. You can structure a document countless ways to represent that query, all of them would likely be different based on the purpose of the app.