- https://stackoverflow.com/questions/5373198/mongodb-relationships-embed-or-reference
- https://www.tutorialspoint.com/mongodb/mongodb_relationships.htm
- https://www.studytonight.com/mongodb/relationships-in-mongodb
- https://docs.mongodb.com/manual/applications/data-models-relationships/
- http://blog.markstarkman.com/blog/2011/09/15/mongodb-many-to-many-relationship-data-modeling/