Social network DB model in MongoDB
According to above mentioned description it seems that story comments is a separate collection which will encapsulate comments related to several stories along with story Id.But that approach will cause a performance overhead since it will require multiple queries for retrieval of information.As an alternate solution comments can be stored into stories collection as embedded documents which will consume less time duration of retrieval of comments
Embedded documents are alternative to joins in MongoDB
For more detailed description regarding embedded documents please refer the documentation as mentioned in following URL

https://docs.mongodb.org/manual/tutorial/model-embedded-one-to-many-relationships-between-documents/