implementation.rb

class Accounting::LedgerItem < ApplicationRecord
  acts_as_ledger_item

  belongs_to :recipient, polymorphic: true
  belongs_to :sender, polymorphic: true

  has_many :line_items

  # # specific line items

  attribute :status, :string, default: 'open'
  attribute :issue_date, :date, default: Date.current
  attribute :period_start, :date, default: Date.current

  def ordered_items
    TypedCollectionProxy.new(self, :line_items, Accounting::LineItems::OrderedItem)
  end

  def discounts
    TypedCollectionProxy.new(self, :line_items, Accounting::LineItems::Discount)
  end

  def fees
    TypedCollectionProxy.new(self, :line_items, Accounting::LineItems::Fee)
  end

  def payments
    TypedCollectionProxy.new(self, :line_items, Accounting::LineItems::Payment)
  end
  
  
  class TypedCollectionProxy

    delegate *Array.public_instance_methods, to: :array

    def initialize(base, attribute, type)
      @base = base
      @attribute = attribute
      @type = type
    end

    def build(attribute = {})
      @base.send(@attribute) << @type.new(attribute)
    end

    def array
      @base.send(@attribute).select { |li| li.type.safe_constantize == @type }
    end

  end
end