Intro to Modules Lab

Objectives

1. Recognize "code smells" that indicate the need for refactoring.

2. Use modules to refactor away repetitious code.

Overview

In this lab, we have an Artist class and a Song class. Artists have many songs and an individual instance of the Song class belongs to an artist. Artists and Songs also have some familiar class methods that keep track of all of the instances of the class, count those instances and clear or reset them.

Open up the lib directory and spend some time reviewing the code in artist.rb and song.rb. Keep reading the code until you feel you understand what each method is doing.

Notice that there are behaviors that are shared between both of these classes. For example, both classes have .count and reset_all class methods. Consequently, both classes have the same exact code. As programmers, you may recall, we are lazy. We don't like to repeat ourselves. We like to keep it DRY (Don't Repeat Yourself). In this lab, we'll be identifying repetition and building modules to extract it out. Then, we'll use the extend and include keywords to lend the functionality of our modules to our Artist and Song classes.

Configuring our Environment

Instead of requiring individual files within one another, as you may have noticed we did in the previous two code along exercises, we created an environment file to handle those requirements for us. Because the environment.rb file is read and loaded into memory in line order, if Artist requires Memorable, then Artist must be required after requiring Memorable. Memorable must be loaded first, so that as Ruby when loads Artist, Memorable::ClassInstances is already defined. Our spec_helper file, which is required by each individual spec file, required only this config/environment.rb file, instead of each and every file from the lib directory. As we start to build larger and more complex programs, it begins to make sense to handle all of our requirements in one place.

Instructions

A Note on Refactoring Practices

We use TDD (test-driven development) for a reason. We write tests to define the desired behavior of our program so that we can write clean, beautiful code. Such code usually isn't the code you write the first time around. The code you first write is the code that makes your program work, the code that gets those tests passing. Then, we refactor our code to make it clean, DRY, and easy to understand. This is where our tests come in. We write thorough tests that cover all of the aspects of our code's desired behavior. We can first write code that passes those tests and then break our code, fail our tests, write better code and pass our tests again.

This is called the red, green, refactor pattern. First tests fail, then you write bad code to get them to pass, then you refactor that bad code into good code. In this lab, you'll start by running the test suite. You'll see that all of the tests pass. Then, we'll break that code in order to refactor it, write better code and get our tests passing again. Remember, don't be afraid of broken code! Broken code is the status quo in programming. Your job is often to break something to make it better. Embrace broken code.

Step 1: Class Methods

First, run the test suite. Wow, we're passing all of our tests! Okay, now let go of those passing tests because we are about to break our code.

The first area of refactoring we'll be attacking are the class methods. Notice that both the Song and Artist class have .count and reset_all class methods. Instead of repeating the same exact code in both classes, let's extract these class methods into a module that we can extend into the classes.

Ready to break your code? Comment out the reset_all and count methods in the Song and Artist class. Run your test suite. Phew! Okay, we did it. That wasn't so bad, was it?

The Memorable Module

Let's define our module. Create a concerns folder inside lib. This is where we'll store our modules. It is a common practice to create a folder called concerns that holds modules that will be used across classes in an object oriented Ruby project.

Inside the concerns folder, create a file called memorable.rb. Open up that file and define a module:

Inside here, define your reset_all and count methods.

Important! Remember to add require_relative '../lib/concerns/memorable' to your environment file before running any tests. We've already provided that line for you in fact! All you have to do is un-comment it out. :)

Once you define the two class methods mentioned above inside of the Memorable module, use the extend keyword to extend those methods, as class methods, into both the Artist and Song class. Refer to the previous code along exercise for help. Remember that the self keyword is omitted when defining class methods inside modules. The extend keyword is responsible for defining the method as a class method vs. an instance method (which would use the include keyword).

Now you're ready to run your test suite again. Get all those tests back to passing before you move on. Once your tests are passing, make sure you delete the commented-out reset_all and count class methods from your Song and Artist class. You don't need them anymore.

Advanced: The find_by_name Method

Before we build the module to house this method, let's talk a bit about it. In an upcoming unit, we'll be introducing databases. You'll learn how to connect your Ruby programs to a database and use that database to store information––even Ruby objects! Moving forward through this course, you'll be building web applications that are connected to databases that store users' information and the information pertinent to the app. Let's think about a common example:

Let's say you're working on an app that serves as an online store, connecting users to everything from books to movies to shoes to stereo equipment, you name it. We'll call this app "Nile" (definitely not inspired by another online market-place named after a famous river). Such an application needs to store the items it has for sale as well as the information of the user who logs in to go shopping. Consequently, every time a user logs in, or searches for an item, or purchases an item, we have to retrieve information from a database. One of the most common ways you'll be doing that is to use methods like find_by_name or find_by_email or find_by_product_id or...you get the idea. We'll be learning much, much more about this later. Here, we're building a simple find_by_name method that introspects on a class's .all class method and extracts the instance of the class with a certain name.

Okay, back to your regularly scheduled programming:

The Findable Module

In lib/concerns, create a file, findable.rb. In this file, define a module: Findable. This module should define the method, find_by_name. This method will be used as a class method. Something like this:

Artist.find_by_name("Adele")
#=> #<Artistx038230sdcmdn3872>

Extract the code from the find_by_name methods that you'll see in the Artist and Song classes and place it inside the Findable module's find_by_name method.

Remember that we need to keep the content of this method abstract. So, inside the Artist class, a find_by_name method might look like this:

class Artist

  @@artists = []

  attr_accessor :name

  def initialize(name)
    @name = name
  end

  def self.all
    @@artists
  end

  def self.find_by_name(name)
    @@artists.detect {|a| a.name == name}
  end
end

Inside the Findable.find_by_name method, we can't use a class-specific class variable like @@artists, because our method would break when included in any class that didn't define such a variable.

Is there a way to reference the collection of all of the instances of a class, without specifically referencing class variables that are only defined in certain classes?

Step 2: Instance Methods

Let's go back to our Song and Artist class and take a look at another example of repetition, this time with instance methods. The to_param instance method is repeated in the Song and Artist class. Another great candidate for refactoring!

Go ahead and comment out the to_param method in both the Song and Artist class. Run your test suite again and see those broken tests!

Okay, now we're ready to define our module.

The Paramable Module

Create a new file in your concerns directory called paramable.rb. Define your module here:

Build the to_param method inside your module and use the include keyword to include the Paramable module in both the Song and Artist class. Once you get your tests passing again, go ahead and delete the commented-out to_param method from the Song and Artist classes.

Important! Remember to add require_relative '../lib/concerns/paramable' to your environment file before running any tests. We've already provided that line for you in fact! All you have to do is un-comment it out. :)

Advanced: The to_param Method

To understand the concept of a parameter, let's take a look at an example URL: www.facebook.com/your-name.

The "your-name" part of the above URL might be referred to as a slug. Another term for this section of a URL is "parameter" or "param". One common task you'll undertake as a web developer is to take a Ruby object, such as an instance of a User class, and make a URL out of it. For example, let's say we have a database full of instances of a User class. When an individual user signs in to our app, we might want to show them their very own profile page. To do so, we would have to write a method that takes their name and turns it into a slug or parameter that could be tacked onto a URL.

Don't worry too much about this use-case for now. We'll be learning much, much more about connecting our Ruby programs to the web later on. For now, just understand the general purpose of having a method like the to_param method.

Bonus: Refactoring the .initialize Method

Recognizing Repetition

Let's take a look at the .initialize methods of both the Song and Artist class:

lib/song.rb

 def initialize
    @@songs << self
  end

lib/artist.rb

def initialize
    @@artists << self
    @songs = []
  end

See the repetition? Both methods push the instance on which they are being called, i.e. self into an array stored inside a class variable.

In song.rb we have:

@@songs << self

In artist.rb we have:

@@artists << self

This is pretty similar, although not exactly the same. However, it is repetitious enough to be giving off a code smell. In order to refactor it, however, we first have to get rid of any code that is specific to the class. In this case, we need to abstract away the literal reference to the @@songs and @@artists class variables.

Abstracting Away Repetition

Lucky for us, we already have class methods that wrap these class variables:

lib/song.rb

lib/artist.rb

def self.all
    @@artists
end

Let's begin by refactoring the content of both .initialize methods to use the <ClassName>.all class method instead of literal class variables. How can we programmatically access the class of the instance that we are operating on inside the .initialize method? Take a look below:

lib/song.rb

def initialize
    self.class.all << self
end

lib/artist.rb

def initialize
    self.class.all << self
    @songs = []
end

Remember that .initialize is an instance method. So, inside .initialize, self refers to the instance of the class on which you are operating. But .all is a class method. We would normally call it like this:

Artist.all

or

Song.all

So, to call the .all class method from inside the .initialize instance method, we can call self.class inside .initialize.

Take a quick look at this reminder of how .class works:

new_song = Song.new
new_song.class
 => Song

So, we can call self.class.all inside .initialize and it will be just as if we called Song.all or Artist.all. Only this way, our code is abstract. It doesn't explicitly reference Song or Artist class, so it is more flexible.

Now we have two .initialize methods that contain identical lines of code. We're ready for the next refactoring step––modules.

Extracting Repetition

Before we build a brand new module to house this code from our .initialize methods, let's stop and think. What is the responsibility or the behavior of the code we are trying to extract? This is code that is responsible for telling a class to keep track of its own instances. This code really goes hand in hand with the .count and .reset_all class methods that we already extracted into the Memorable module. It makes sense, therefore, to extract this code into that same module.

But wait (you might be thinking), isn't that module extended into our Song and Artist class in order to offer its methods as class methods? Isn't .initialize an instance method? How can we put class methods and instance methods in the same module? Read on to learn the answer...

Nesting Modules

We can nest sets of modules within one another and then include or extend individual modules as needed. Let's take a look:

module Memorable
  module ClassMethods
    def reset_all
      self.all.clear
    end

    def count
      self.all.count
    end
  end

  module InstanceMethods
    def initialize
      # some more code coming soon!
    end
  end
end

Then, in order to include or extend as needed, we use the include or extend keyword in the following manner:

In both the Song and Artist classes:

extend Memorable::ClassMethods
include Memorable::InstanceMethods

The Parent::Child syntax is called namespacing.

Okay, we're almost done. We need to fill out the content of the .initialize method in the module.

The .initialize methods in our Song and Artist classes share the following line:

def initialize
  self.class.all << self
end

This is the code that will go into the new .initialize method of our module:

module Memorable
  module ClassMethods
    def reset_all
      self.all.clear
    end

    def count
      self.all.count
    end
  end

  module InstanceMethods
    def initialize
      self.class.all << self
    end
  end
end

There's just one more step. Look back at the original .initialize method of the Artist class:

class Artist
  ...

  def initialize
    self.class.all << self
    @songs = []
  end

In the Artist class, the initialize method is also responsible for setting the @songs instance variable equal to an empty array. We need to hang on to this behavior, even as Artist instances grab the rest of the .initialize from the Memorable::InstanceMethods module.

Remember our super keyword from the inheritance code along exercise? The super keyword, placed inside a method, will tell that method to look up its behavior in the method of the same name that lives in the parent, or super, class. A method that includes the super keyword will execute any code placed inside the super class' method of the same name, and then execute any code inside the child class' method.

When we include a module in a class, we are really telling that class to inherit methods from that module.

So, we can use the super keyword to tell our Artist's .initialize method to use the code in the Memorable::InstanceMethods module's .initialize method and also to use any additional code in the Artist's .initialize method. Take a look:

class Artist
  ...

  def initialize
    super
    @songs = []
  end

Conclusion

Phew! That was some complex stuff. It's okay if you didn't understand everything covered in this lab. There were a few advanced and bonus sections that we threw in there to challenge you and make you think. Don't skip over them, even if you can't follow everything they discuss. It's important to plant the seed of some of these more complex topics––it will make them easier to understand later on when you're ready to go deeper into Ruby programming.

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