app_controllers_bar_controller.rb

class BarContoller
  def update
    # ...
    SomeExternalModule::SomeClassInTheModule.my_method
    # kaboom:
    # NoMethodError (undefined method `my_method' for #<Class:0x007fd3964eb468>):
  end
end

app_models_foo.rb

module SomeExternalModule
  class SomeClassInTheModule # which inherits ActiveRecord::Base
    def self.my_method
      # do whatever here...
    end
  end
end

as_an_eval.rb

#? lxsameer's suggestion
SomeExternalModule::SomeExternalClass.class_eval do
  def my_method
    # do whatever here...
  end
end