Reading

Read the following chapters from The Pragmatic Programmer (in our library):

• Software Entropy
• The Evils of Duplication
• Orthogonality
• Decoupling and the Law of Demeter
• Evil Wizards

Read the following chapters from Clean Code (in our library):

• Chapter 1: Clean Code
• Chapter 2: Meaningful Names
• Chapter 3: Functions
• Chapter 4: Comments
• Chapter 5: Formatting
• Chapter 8: Boundaries
• Chapter 10: Classes
• Chapter 12: Emergence
• Chapter 17: Smells and Heuristics

Also read:

• Keep It DRY, Shy, and Tell the Other Guy Archive
• GRASP (object-oriented design) Archive
• Single Responsibility Principle Archive

Read the following chapters from our book Growing Rails Application in Practice:

• Dealing with fat models
• Extracting service objects

Discuss with your mentor what you took away from each topic.

Exercise

We're building an e-commerce app where users can create and view invoices.

This is our current model:

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class Invoice < ApplicationRecord
  has_many :items
  validates_presence_of :recipient_address, :number
end

class Invoice::Item < ApplicationRecord
  belongs_to :invoice
  belongs_to :product
  validates_numericality_of :units
end  

class Product < ApplicationRecord
  validates_presence_of :description
  validates_numericality_of :unit_price
end

This is a view that shows an invoice:

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%h1
  Invoice
  = @invoice.number

%h3 Recipient
= @invoice.recipient_address

%h3 Items

%table
  %tr
    %th Description
    %th Quantity
    %th Item total

  - @invoice.items.each do |item|
    %td= item.product.description
    %td= item.units
    %td= item.units * item.product.unit_price
    
  %tr
    %th
      Invoice total
    %td(colspan=3)
      = @invoice.items.sum { |item| item.units * item.product.unit_price } * 1.19

How would you judge the quality of this code?
Can you apply what you learned and improve the model and the view?