The Basics of Creating and Using Ruby Modules

Ruby on Rails is a wonderful open source full-stack web application framework favoring convention over configuration.  With reusable, easily configurable components normally used for creating applications, building applications in Rails is faster and easier resulting in improved productivity and business growth. It is gaining traction with developers as it is flexible, scalable, and easy for web developers to write and maintain applications. 

Ruby on Rails lays emphasis on to use known engineering patterns and principles in order to reduce the workload when building web applications. Even though there are multiple methods to solve a programming challenge, Ruby professes to rely on commonly used patterns making Rails websites easier to maintain and upgrade. Open source libraries with smaller blocks of Rails code solving particular problems are commonly used to reduce development overhead. With the help of such code blocks, developers can insert stable, well-tested gems instead of spending time to write the same kinds of codes. One of the most powerful tools in a Ruby programmer’s toolbox is the module. Ruby Modules provide a structure to collect Ruby classes, methods, and constants into a single, separately named and defined unit.

This is useful so you can avoid clashes with existing classes, methods, and constants, and also so that you can add (mix in) the functionality of modules into your classes.

Creating a Module in Ruby

Creating a module is similar to creating a class, except you use the module keyword instead of the class keyword.

module MyFirstModule
  def say_hello
    puts "Hello"
  end
end

Modules cannot be instantiated. To use a module, you must either include or extend it within a class.

Using include and extend

• include

Mixes the module’s methods in as instance methods of the class.

• extend

Mixes the module’s methods in as class methods of the class.

Example

module ReusableModule
  def module_method
    puts "Module Method: Hi there!"
  end
end

class ClassThatIncludes
  include ReusableModule
end

class ClassThatExtends
  extend ReusableModule
end

puts "Include"
ClassThatIncludes.new.module_method
# => "Module Method: Hi there!"

puts "Extend"
ClassThatExtends.module_method
# => "Module Method: Hi there!"

Other Important Concepts About Modules in Ruby

1. Method Lookup Basics:

module M
  def report
    puts "'report' method in module M"
  end
end

class C
  include M
end

class D < C
end

obj = D.new
obj.report

Output:

'report' method in module M

How Method Lookup Works

When a Ruby object receives a message (method call), it searches for the method in this order:

1. The objects class (D)
2. Modules included in the class (D)
3. The superclass (C)
4. Modules included in the superclass (M)
5. Continue up the inheritance chain

In this case:

• D does not define report
• C does not define report
• C includes module M
• M defines report, so Ruby executes it

2. Defining the Same Method More Than Once

If a method is defined multiple times, the last definition takes precedence. This applies to both classes and modules.

module InterestBearing
  def calculate_interest
    puts "Placeholder! We're in module InterestBearing."
  end
end

class BankAccount
  include InterestBearing

  def calculate_interest
    puts "Placeholder! We're in class BankAccount."
    puts "And we're overriding the calculate_interest method!"
  end
end

account = BankAccount.new
account.calculate_interest

Output:

Placeholder! We're in class BankAccount.
And we're overriding the calculate_interest method!

The class method overrides the module method.

3. Mixing in Multiple Ruby Modules with the Same Method Name

module M
  def report
    puts "'report' method in module M"
  end
end

module N
  def report
    puts "'report' method in module N"
  end
end

class C
  include M
  include N
end

c = C.new
c.report

Output:

'report' method in module N

The last included module wins because it appears first in the method lookup path.

4. Including a Ruby Module More Than Once

class C
  include M
  include N
  include M
end

c = C.new
c.report

Output:

'report' method in module N

Explanation

Re-including a module has no effect if the module is already in the lookup path. Ruby does not move it to the top again. The most recently included new module (N) still takes precedence.

5. Using super with Ruby Modules

The super keyword allows you to call the next method up the method lookup chain.

module M
  def report
    puts "'report' method in module M"
  end
end

class C
  include M

  def report
    puts "'report' method in class C"
    puts "About to trigger the next higher-up report method..."
    super
    puts "Back from the 'super' call."
  end
end

c = C.new
c.report

Output:

'report' method in class C
About to trigger the next higher-up report method...
'report' method in module M
Back from the 'super' call.

What Happens Here

• Ruby first executes report in class C
• super calls the next report method in the lookup chain (from module M)
• Control then returns back to the class method

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