I had problems getting the plugin to work with Mongoid out of the box.
I had to override OauthConsumersController#load_consumer with this:
throw RecordNotFound unless OAUTH_CREDENTIALS.include?(consumer_key)
deny_access! unless logged_in? || consumer_credentials[:allow_login]
@token=current_user.consumer_tokens.where(type: @consumer).first if logged_in?
It seems to me that this should work with ActiveRecord just as well as the original code, but I haven't tested it.