Creating the architecture of a cross-platform mobile app with RubyMine and RubyMotion

I'm creating a hybrid cross-platform app (i.e. iOS, Android, iWatch) with Ruby on Rails, RubyMine and RubyMotion. I'm using PostgreSQL as the back-end.

My goal is to minimize code duplication and maximize code use. So, I'm considering using a Ruby gem or Rails engine to store my model and controllers. Then I'll create separate RubyMine projects for each mobile interface so that each contains only view code for that platform.

Which architecture layers, design patterns, Ruby on Rails-specific features and RubyMine functionality should I use?

Feel free to add or improve upon any detail I'm missing.

Thanks.