This past week was a bit hectic for me considering the pace picked up at the Flatiron School and on top I had to prepare for a meetup. I managed to make it through the week and after getting 12 hrs of much needed sleep on Saturday I went about reviewing the recorded lectures. By revisiting these lectures I had several ‘Aha’ moments and would like to share them. I apologize in advance if you already know this but for me…

1. Modules should have abstract values

Don’t rely on literal values! A major problem with relying on literal values is scope. In the example below on line 4 the @@all variable should be encapsulated in a method.

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module Rankable
  module ClassMethods
    def top_5
      @@all.sort_by {|s|s.rank}[0..4]
    end
  end
end

The fix is to encapsulate @@all in a method resulting in the following code.

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module Rankable
  module ClassMethods
    def top_5
      self.all.sort_by {|s|s.rank}[0..4]
    end
  end
end

2. Order Matters

When listing the files to load up it should be

1. Gems 
2. Concerns(Modules) 
3. Models(Classes)

The following code would break because the module line 3 is loaded after the model(Student Class)

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require 'sqlite3'
require_relative '../lib/student' #class Student
require_relative '../lib/concerns/persistable' #module Persistable

The fix is to load it in the correct order.

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require 'sqlite3'
require_relative '../lib/concerns/persistable' #module Persistable
require_relative '../lib/student' #class Student

3. Be Weary of Class Variables(@@)

Class variables can leak due to inheritance. For example in the following code when a new Dog instance is created the @@breeds will be overwritten to [:affenpoo].

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class Dog
  @@breeds = [:poodle, :pug]
end

class Poodle < Dog
  @@breeds = [:affenpoo]
end

To avoid possible leaks, class CONSTANTS should be used. The following would fix the problem above and when a new Dog instance is created the BREEDS constant in the Dog class will remain [:poodle, :pug]. Likewise the BREEDS constant in the Poodle class will remain [:affenpoo].

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class Dog
  BREEDS = [:poodle, :pug]
end

class Poodle < Dog
  BREEDS = [:affenpoo]
end

4. All Methods Must Have a Receiver

All methods must have a receiver and if not defined it is self.

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class Movie
  attr_accessor :name
  def save
    DB.execute("INSERT INTO movies(name) VALUES (?)", name)
  end
end

In line 4 the last argument ‘name’ is not a variable. The way ruby determines this is by first looking for a variable called ‘name’. If not defined it then looks for method called name which is defined on line 2 “attr_accessor :name”.

5. Found Another Tool – The Tap Method

What the tap method does is yields x to the block and then returns x.

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def self.new_from_db(row)
  s=self.new
  s.id=row[0]
  s.name=row[1]
  s.tagline=row[2]
  s
end

Using the tap method

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def self.new_from_db(row)
  self.new.tap do |s|
    s.id=row[0]
    s.name=row[1]
    s.tagline=row[2]
  end
end