I have been using MongoDB for a few years and really like it. One useful feature is ability to store complex data types such as Hashes or Arrays in fields. Actually MongoDB itself does not support Hashes but you can do it using ODM like Mongoid. It is much eaiser than serializing complex structure and storing it as string in the DB.
The challenge comes when you have to manually edit these fields. You know those situations where you get a call from a customer who made a mistake and you have to fix the data. We are using RailsAdmin which works very well with Mongoid.
For editing Arrays you can create a RailsAdmin custom field following these instructions. But Hashes are tricky and you often have to edit it as a string, manually modifying complex data structure.
Instead of Hash why not create separate model and embed it inside the main document?
I can now use RailsAdmin to edit Foo and inside it I can edit various Bars. I am not ready to completely give up on Hashes but for certain things I am starting to really like embedded documents. Embedding it in parent document makes it very easy to access and just loop through Foo.bars.each