I used to build Ruby on Rails apps with MySQL.
MongoDB currently become more and more famous and I am now starting to give it a try.
The problem is, I don't know the underlying theory of how MongoDB is working (am using mongoid gem if it matter)
So I would like to have a comparison on the performance between using MySQL+ActiveRecord and model generated by mongoid gem, could anyone help me to figure it out?
I don't know much of the underlying theory. But this is the advice I got: only use MongoDB if you run it across multiple servers; that's when it'll shine. As far as I understand, the NoSQL movement appeared in no small part due to the pain of load-balancing relational databases across multiple servers. So if you're hosting your application on no more than one server, MySQL would be the preferred choice.
The good people over at the Doctrine project recently wrote a quite useful blog post on the subject.