[Rails] Using Mongoid with ActiveRecord

I'm trying to use mongodb and postgresql simultaneously with Rails
3.1. I believe I have everything set up correctly.

What I'm interested in knowing is how to get rails to flip-flop back-
and-forth between mongoid and activerecord.

For example, if I want to generate an ActiveRecord model after using
mongoid, rails automatically defaults to the Mongoid gem.
invoke mongoid
create app/models/blogpost.rb
invoke rspec
create spec/models/blogpost_spec.rb

How would I get rails to use ActiveRecord instead, and vice versa?

Thanks in advance

Mike