I just began a new rails project and wanted to use MongoidDB thru the Mongoid gem. Following the instructions on the Mongoid site, I added the following lines to my G
You're probably setting some config.active_record settings somewhere in your config folder (development.rb, test.rb, etc). You'll want to comment out these settings since you're not using ActiveRecord.
You can do something like grep -r active_record config/ in your project folder to find files referencing active_record.
Update: Make sure you have removed the require 'rails/all' line from config/application.rb as well. The new require lines you added are supposed to replace that line. Otherwise, rails/all still loads ActiveRecord.