I think these three are the most popular non-relational db:s out there at this moment.
I want to give them a try, but I wonder which one of these are most suitable for Rails when it comes to Gem, documentation and tutorial support.
Eg. if I install a very good gem that is for Rails but this just use AR and mongodb, then it would be a pity I didn't use mongodb.
How many gems are supporting each one of these databases?
Which one is the most popular and main-stream in ruby/rails community, thus has more online documentations/tutorials?
Which one offers tight integration of Rails?