Can someone who is using MongoDB and a RDBMS at the same time tell me the exact use cases where MongoDB is fitter? Structured logs? Caching? Sessions? And why MongoDB does it better?
My previous company store coupon code on it (referrals). I prefer a hash nosql for that though (cassandra) but for a one man shop it's overkill.
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IIRC postgresql have their own document storage like MongoDB. I just never had to use such thing because I guess I'm too OCD about how my data suppose to look like.
I use MongoDB for everything where I don't know the data structure before hand. MongoDB is very good in indexing unknown data structures and makes them searchable. Also, when I build prototypes I often use MongoDB.