Please provide a $populate stage that allows to resolve single referenced documents.
Internally it could use the combination of $lookup and $unwind:

https://stackoverflow.com/questions/41482559/how-to-return-single-object-instead-of-array-in-lookup-of-mongodb

Related:

https://stackoverflow.com/questions/37793844/mongodb-how-to-resolve-dbref-on-client-side

https://jira.mongodb.org/browse/SERVER-22497