What are SingletonMethods and InstanceMethods in Ruby

Question

I see a lot of stuff

include ActiveRecord::XXXX::InstanceMethods
extend ActiveRecord::XXXX::SingletonMethods

I am unaware of the property or there working, just wanted a easy to understand answer. if it has a good reason to be used.

Answer 1

Instance methods are methods you define within a class that are sent to INSTANCES of that class. Singletom or class methods are methods defined within a class that are sent to the CLASS object. As a simple example:

Class A 
def instance_method
 puts "Hi, I'm an instance method"
end
def self.singleton_method 
  puts "Hi, I'm a singleton method"
end
end

you call them like this:

singleton:

A.singleton_method

instance_method:

a = A.new
a.instance_method

Answer 2

It's a not-written convention usually adopted when you want to encapsulate features/extensions in a Ruby module intended to be used as a Mixin.

module Bar
  def beta
    puts "I'm beta"
  end
end

class Foo
  include Bar

  def alpha
    puts "I'm alpha"
  end
end

Foo.new.beta
# => "I'm beta"

Sometimes the Mixin is a simple as providing some instance methods, sometimes else you need to extend the original class with both instance and class methods. The following syntax in Ruby is invalid.

module Bar
  def beta
    puts "I'm beta"
  end

  def self.class_beta
    puts "I'm class-beta"
  end
end

class Foo
  include Bar

  def alpha
    puts "I'm alpha"
  end
end

Foo.new.beta
# => "I'm beta"

Foo.class_beta
# => invalid

That said, you can't include a module with method defined with self.

For this reason, it's a common pratice to split the module in two submodules and use the self.included hook.

module Bar

  def self.included(base)
    base.extend         ClassMethod
    base.send :include, InstanceMethods
  end

  module ClassMethods
    def class_beta
      puts "I'm class-beta"
    end
  end

  module InstanceMethods
    def beta
      puts "I'm beta"
    end
  end

end

class Foo
  include Bar

  def alpha
    puts "I'm alpha"
  end
end

Foo.new.beta
# => "I'm beta"

Foo.class_beta
# => "I'm class beta"

Also, check out this great article written by Yehuda Katz.