Single Responsability Principle: A class should have one and only one reason to change, meaning that a class should have only one job.

srp.rb

# Violates SRP
class Invoice
  def initialize(items, client)
    @items = items
    @client = client
    @total = 0
  end

  def generate
    @total = @items.inject(0) { |sum, i| sum + i }
  end

  def send_email
    Mailer.deliver(
      from: 'my_solid_company@email.com',
      to: @client.email,
      subject: 'Your new invoice was created!',
      invoice: self
    )
  end
end

# Enforces SRP
class InvoiceGenerator
  def initialize(items, client)
    @items = items
    @client = client
    @total = 0
  end

  def generate
    @total = @items.inject(0) { |sum, item| sum + item.price }
    self.save
  end
end

class InvoiceMailer
  def initialize(invoice)
    @invoice = invoice
  end

  def send_email
    Mailer.deliver(
      from: 'my_solid_company@email.com',
      to: @client.email,
      subject: 'Your new invoice was created!',
      invoice: @invoice
    )
  end
end