How to create and use a module using Ruby on Rails 3?

I am using Ruby on Rails 3 and I would like to move some custom and shared code in a module.

1. What syntax should I use to write the module code?
2. In which folder of my application I have to place the module file?
3. How I have to include that module in one or more controller classes?
4. What other action, if any, do I have to use the custom module anywhere in my application?
5. How can I call methods in the module from my application?

Thanks in advance.

To 1. A module is created/opened by simply saying:

module MyModule
  def first_module_method
  end
end

To 2. The lib folder. If you want to organize your modules in the lib folder, you can put them into modules themselves. For example, if you wanted a subfolder super_modules your modules would be defined as follows:

module SuperModules
  module MyModule
    def first_module_method
    end
  end
end

To 3./5. When including the module in a class you can simply call the modules methods as if they were defined within the class:

class MyClass
  include MyModule
  def some_method
    first_module_method #calls module method
  end
end

To 4. Frst, make sure that your module is really needed in every class of your application. If it isn't it makes sense to only include it where it is need so as not to bloat the classes that don't need it anyways. If you really want the module everywhere, include look at the class hierarchy of your classes in the app. Do you want the module in all models? You could open ActiveRecord::Base and add add your module there.

A>1. You can use the same syntax as any other ruby class. For instance, I'm defining a VehicleClassifer module which is going to use the classify_vehicle method to classify a vehicle based on the number of wheels it receives as an input.

module VehicleClassifer
  def classify_vehicle(number_of_wheels)
    VehicleType.where("number_of_wheels = ?", number_of_wheels)
  end
end

A>2. Modules are usually stored in the /lib folder.

questions 3,4,5 have more or less the same answer. you can use

class SomeController < ApplicationController
  include VehicleClassfier

  def index 
    classify_vehicle(4)  
  end
end

in the class you're using the module and you will have access to all the module's methods.

Also, In case you need to use a module through out your app, you can include it in your application controller.