我没有使用过json builder。我用ruby做的，希望你能翻译成builder格式。

# Grouping all records by expense_type_id, so that we can sum
records = @expenses.group_by{ |expense| expense[:expense_type_id] }
records.map do |expense_type_id, items|
  # Initializing total as zero, so that we can sum inside loop. We can sum later in separate loop, but better to avoid many loops
  total = 0
  attributes = items.map do |item|
    bill = expense.billed.to_f
    total += bill
    {
      amount: bill, 
      id: expense.id,
      expense_type_id: expense_type_id
    }
  end
  # attributes will contain all formatted records, and lastly we insert our total
 attributes << {amount: total, id: 'Subtotal', expense_type_id: nil }
end.flatten.compact # Since we are using double map, we get array of arrays, so need to flatten

我会采用面向对象的方法，通过定义一个名为ExpenseType的类（也许你的应用程序中已经有了这个，因为我看到了一个变量expense_type_id）。

Expense.all.group_by(&:expense_type_id).map do |type_id, expenses|
  ExpenseType.from_list(type_id,expenses).to_json
end

现在，ExpenseType示例数组可以负责聚合和格式化它们的json表示，类似于...

# expense_type.rb
class ExpenseType
  attr_accessors :type_id, :list

  def self.from_list(type_id, list)
    new(type_id, list)
  end

  def initialize(type_id, list)
    @type_id = type_id
    @list = list
  end

  def to_json
    # skipping the detail here
    # make a json representation of the list of expenses with this expense_type_id by calling .to_json on each
    # followed by a json representation of the sum for this expense_type_id
  end
end

最后，每个Expense对象都知道如何将自己表示为json：

# expense.rb
class Expense < ApplicationRecord # I assume it's a model in a Rails app
  def to_json
    # return json representation of a single expense
  end
end