Refactor problem

invoice.rb

# Introduced these two classes to the students and asked them to refactor/improve the `calculate_balance` method.
# We initially suggested maybe breaking up the method in to smaller ones.

class Invoice < ActiveRecord::Base
  belongs_to :account

  def calculate_balance
    if display_ids.present?
      items = account.items.visible.where("id IN (#{display_ids})").oldest_first
    else
      items = account.items.visible.where("created_at > ? AND created_at < ?", starts_at, ends_at).oldest_first
    end

    total = 0

    if account.due_at < Time.zone.now
      total += account.late_fee
    end

    items.each do |x|
      total += x.amount
    end

    if account.due_at < Time.zone.now
      self.late = true
    end
    self.total = total
    save
  end
end

class Account < ActiveRecord::Base
  has_many :invoices
  attr_accessible :due_at
end