Shopping cart app

An app that allows you to add products, define discount rules for each product, and put them into the cart!

Table of contents

• DEV environment
• Database
• Logic

Setting up the environment

1. Get Ruby 3.3.3
2. bundle install
3. rails db:create && rails db:migrate && rails db:seed

Nothing else is needed. Bootstrap is already in the project files.

Database

I went for UUID as cart ID for security reasons. Almost all the validation is on the DB level (in models too) to assure only valid data is saved into DB no matter what. We don't want nil prices or amounts.

A few words about logic

Each product can have zero or more discount rules. Cart holds CartItems, and each Product group is a CartItem.

DiscountRule attributes

• discount_type - type of discount based on which we calculate the product group price.
• amount - amount of discount in cash/percent, depending on type. Always greater than 0.
• min_quantity - minimum product quantity from which the discount is applied. Greater than or equal to 0 (we may want to discount the product regardless of its amount)
• status - status of the rule active | inactive. Each product can have exactly 1 active pricing rule.

There are 3 types to choose from for the DiscountRule:

• buy_one_get_one_free - every other product is free of charge. min_quantity and amount are not taken into account during the calculation
• price - price discount on every product
• percentage - percentage discount on every product

Prices and percentage have to be passed as integers: 5% ➡️ 50, 10€ ➡️ 100. They're displayed that way in forms, but the user sees the normal user-friendly price format, e.g €3.11.

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We have 2 methods to manage the Cart

• add - to add items and change their quantity. We can increase or decrease the amount by Quantity input. ShoppingCart::Items::Manager is called here. The amount increases when we pass positive value and decreases otherwise.
• remove - to remove particular item completely. ShoppingCart::Items::Destroy is called.

To manage CartItems

• Manager - called when we add an item to the cart. If the item is already in cart, it tries to update it. Otherwise - creates a new one.
• Create - as the name says - creates a new CartItem and calculates its discount (if any applicable).
• Update - if the current CartItem.quantity + :quantity param from the input is positive - updates the value. If it's negative - call Destroy class.
• Destroy - destroys the CartItem. No additional logic here.

classDiagram
  Manager : +Cart cart
  Manager : +Product product
  Manager : +Integer quantity
  Manager : cart_item()
  Create : +Cart cart
  Create : +Product product
  Create : +Integer quantity
  Create : +discount_amount()
  Create : +discounts_applied()
  Create : +total_price()
  Create : +price_before_discount()
  Create : +discount_applicable?(discount_role)
  Create : +active_discount_rules()
  Update : +Cart cart
  Update : +CartItem cart_item
  Update : +Integer quantity
  Update : +quantity()
  Update : +product()
  Destroy : +Cart cart
  Destroy : +Product product
  Destroy : +cart_item()
  note for Create "Product NOT in cart"
  Manager --> Create
  note for Update "Product IS in cart"
  Manager --> Update
  Update --> Destroy
  note for Destroy "(CartItem.quantity + params[:quantity]) <= 0 || CartItems#remove call"

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Each discount rule has a separate strategy that does the calculation. I've used #ceil for rounding, because it's rounding up (in customer favor), while #round is rounding down 😄

DiscountCalculator::Manager is responsible for delegating to the right strategy based on PricingRule.discount_type. It's called in ShoppingCart::Items::Create#discount_amount and ShoppingCart::Items::Update#discount_amount.

classDiagram
  Manager : +Product product
  Manager : +Integer quantity
  Manager : +DiscountRule discount_rule
  Manager : +product_price()
  Manager : +calculate_discount()
  Manager : +discount_strategy_class()
  BuyOneGetOneFreeStrategy : +Product product
  BuyOneGetOneFreeStrategy : +Integer quantity
  BuyOneGetOneFreeStrategy : +amount()
  PercentageStrategy : +Product product
  PercentageStrategy : +Integer quantity
  PercentageStrategy : +DiscountRule discount_rule
  PercentageStrategy : +amount()
  PriceStrategy : +Product product
  PriceStrategy : +Integer quantity
  PriceStrategy : +DiscountRule discount_rule
  PriceStrategy : +amount()
  Manager --> BuyOneGetOneFreeStrategy
  Manager --> PercentageStrategy
  Manager --> PriceStrategy
  note for BuyOneGetOneFreeStrategy "discount_rule.discount_type == 'buy_one_get_one_free'"
  note for PercentageStrategy "discount_rule.discount_type == 'percentage'"
  note for PriceStrategy "discount_rule.discount_type == 'price'"

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