Idea of new Invoicing design.

idea.rb

class Batch
end

class Invoice < ActiveRecord::Base
  belongs_to :invoicable, polymorphic: true
  belongs_to :invoicer, polymorphic: true

  has_many :invoice_lines

  def amount
    invoice_lines.sum(&:amount)
  end
end

class ParentInvoice < Invoice
end

class ChildminderInvoice < Invoice
end

class InvoiceLine
  attr_accessor :description, :amount_per_unit, :units, :amount
  attr_accessor :line_type
  attr_accessor :order

  # line_type: [:header, :item]
end

class Person
  has_many :invoices, as: :invoicable
  has_many :sent_invoices, as: :invoicer, class_name: "Invoice"
end

class Franchisee
  has_many :sent_invoices, as: :invoicer, class_name: "Invoice"
end

class ParentInvoiceGenerator
  def self.invoice_for_all_parents(year, month)
    people = people_elegilble_for_invoice(year, month)
    people.each { |person| invoice_for_parent(person, year, month) }
  end

  def self.invoice_for_parent(person, year, month)
    invoice = ParentInvoice.new
    #...
  end

  private
  def people_eligible_for_invoice(year, month)
    # Do query here
  end
end

class ChildminderInvoiceGenerator
  def self.invoice_for_childminder(person, year, month)
    invoice = Invoice.new(invoice_type: :childminder)
    #...
    invoice.invoice_lines << InvoiceLine(descrtion:
  end
end