ruby-object-design

Ruby Object Design Expert

Ruby is object-oriented, not class-oriented. The class keyword should be reserved for specific use cases, not used as a default container for code.

The Object Factory Rule

Only use class if you are creating an object factory -- a template for generating multiple objects that encapsulate internal state with behaviors operating on that state.

If your code doesn't create multiple instances with distinct state, a class is the wrong construct.

Decision Tree

Do you need multiple instances with encapsulated state?
|-- YES: Does the object have both state AND behavior?
|   |-- YES -> Class
|   +-- NO (just data) -> Struct or Data
+-- NO: Is this a collection of related functions?
    |-- YES -> Module with `extend self`
    +-- NO: Is this a one-off transformation?
        +-- YES -> standalone method or lambda

Red Flags: When NOT to Use a Class

1. Stateless Utility Buckets

If your class has no instance variables, it's a module pretending to be a class.

# Bad: Class with no state
class StringUtils
  def self.titleize(string) = string.split.map(&:capitalize).join(' ')
end

# Good: Module with extend self
module StringUtils
  extend self
  def titleize(string) = string.split.map(&:capitalize).join(' ')
end

Why module: Clearer intent, can be included, no misleading .new method.

2. Single-Method "Service" Classes

Classes with only initialize + call are often functions in disguise. Ask:

• Will multiple instances exist simultaneously?
• Does the object maintain state between method calls?
• Is initialize + call adding clarity or ceremony?

# Questionable: function in a class costume
class CalculateDiscount
  def initialize(order) = @order = order
  def call = @order.subtotal * discount_rate
  private
  def discount_rate = @order.customer.premium? ? 0.1 : 0.05
end

# Alternative: Module function
module Discounts
  extend self
  def calculate(order) = order.subtotal * discount_rate(order.customer)
  private
  def discount_rate(customer) = customer.premium? ? 0.1 : 0.05
end