Integrating the fb_graph or koala gem with devise+mongoid+omniauth in rails 3

I need Facebook API integration in my rails application, but I can't
figure out how to store the Facebook access token from Omniauth so
that it can be reused later by a Facebook API gem. The Facebook API
gems I am considering is fb_graph or koala (don't really care which),
but I am open to other solutions if there are better alternatives.

A question on Stackoverflow provides a solution which the questioner
(and I) were not able to get to work:

The base code I want to integrate fb_graph or koala into comes from
the official devise example page:
The base I am using integrates devise+mongoid+omniauth:

Any tips on how to integrate fb_graph or koala (or any other facebook
API connection) with this devise+mongoid+omniauth base would be
greatly appreciated.