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As I find in MongoDB [FAQ](http://docs.mongodb.org/manual/faq/fundamentals/#does-mongodb-require-a-separate-caching-layer-for-application-level-caching)

> Does MongoDB require a separate caching layer for application-level caching?

> No. In MongoDB, a document’s representation in the database is similar to its representation in application memory. This means the database already stores the usable form of data, making the data usable in both the persistent store and in the application cache. This eliminates the need for a separate caching layer in the application.

AND

> Does MongoDB handle caching?

> Yes. MongoDB keeps all of the most recently used data in RAM. If you have created indexes for your queries and your working data set fits in RAM, MongoDB serves all queries from memory.


I am not sure whether the current implementation will benefit from the MongoDB caching mechanism. Or do I need to make the `Feed` data type an instance of `MongoDBPersistent`?