State machine gem: undefined method underscore for ActiveModel::Name

If  you encounter this issue:

Failure/Error: ::Mobile::App::Talk.make!
NoMethodError:
  undefined method `underscore' for #<ActiveModel::Name:0x0000000ebae880>

When using state machine, machinist and machinist_mongo, please add this to your app/config/initializers/state_machine.rb:

# It seems that there is no underscore method on ActiveModel name
# It is added here, so it will be accessible and state machine
# can work. It should be removed after this is fixed
class ActiveModel::Name
  def underscore
    to_s.underscore
  end
end

This should be enough to make it work :)