It is sometimes handy to turn on CoreData.SQLDebug in the simulator when debugging a Core Data issue. I have found the output to be very useful in finding the hard to find little bugs that pop-up between restkit and core data. It can also help find performance issues with the database. Unfortunately there isn't any documentation for how to do this in RubyMotion. I found one solution that works to a degree. It requires installing the ios_sim app and forcing the simulator into CoreData.SQLDebug mode after the rubymotion build.
You will need to install ios_sim
$ brew install ios-sim
Then add the following two line as the last lines in your RubyMotion rakefile
# Be sure to substitute your app name and make sure you are using
# the correct path to the app based on simulator version.
ios_sim = `which ios-sim`.strip
sh "#{ios_sim} launch build/iPhoneSimulator-6.1-Development/appname.app --args -com.apple.CoreData.SQLDebug 1"
This will open the simulator in CoreData.SQLDebug mode, and log debug information to the terminal.