Question

I’m missing something stupid. Help?

lib/api.rb

require 'httparty'

module API
  def self.call_api(query)
    base_url = "http://myapi.com"
    return HTTParty.get("#{base_url}/#{query}.json")
  end
end

models/job.rb

require 'api'

class Job
  include API

  def self.all(page=1)
    self.call_api "jobs?page=#{page}"
  end

end

Job::all
NoMethodError: undefined method `call_api’ for Job:Class

If I move my “call_api” directly into the Job class, it works. What am I missing?

Answer 1

The other answers have pointed out your issue you should have extend instead of include:

class Job
  extend API

You might also like to consider using the following pattern which aside from being useful can also help to alleviate the confusion, since you can tell straight away that your class methods are in the ClassMethods module while your instance methods are directly in the MyModule module.

module MyModule

  def self.included(base)
    base.extend(ClassMethods)
  end

  module ClassMethods 
    def my_class_method
    end
  end

  def my_instance_method
  end
end

This uses Ruby’s included callback (which is fired every time a module in included in a class), we redefine the callback (which normally doesn’t do anything), to extend the class in which the module was included with the ClassMethods sub module. This is a very common metaprogramming pattern in Ruby. If you use this pattern you no longer have to worry about extend, just use include:

class MyClass
  include MyModule

then:

MyClass.my_class_method
MyClass.new.my_instance_method

You can also take this pattern a little bit further:

module MyModule
  include ClassMethods

  def self.included(base)
    base.extend(ClassMethods)
  end

  module ClassMethods 
    def my_class_method
    end
  end

  def my_instance_method
  end
end

Notice that the parent module includes its child ClassMethods module directly. This way my_class_method becomes both an instance and a class method:

class MyClass
  include MyModule

MyClass.my_class_method
MyClass.new.my_class_method

You have to be a bit careful if you do that about how you code your method in the ClassMethods child module. But I have found this pattern extremely handy on occasion.