有没有一种简单的方法可以计算 Rails 中的和积？

Question

我有一个 PositionGroup has_many :positions。

每当触摸 position_group 对象时，我想像这样更新 pg.average_price：

# Average Price = ((position1.transaction_price * (position1.volume/total_volume) + 
# position2.transaction_price * (position2.volume/total_volme))

在我的回调方法中，我试过这个：

  def update_average_price
    total_volume = positions.sum(:volume)
    avg_price = positions.sum("transaction_price * (volume/#{total_volume})")
    update_column(:average_price, avg_price)
  end

但是当我检查 avg_price 的值时，一旦存在多个 positions，我得到 0.0。

这是我对这个特定功能的规范：

it "should calculate the Weighted Average Purchase Price of all positions" do
  # Position 3: Price = 20, volume = 200
  # (Position 1 Price * (Position 1 Units/Total # of Units)) +
  # (Position 2 Price * (Position 2 Units/Total # of Units)) +
  # (Position 3 Price * (Position 3 Units/Total # of Units))
  # ( * (100/400)) + ( * (100/400) +  * (100/400)) =
  # (.50 + .75 + ) = .25
  price3 = 20
  pos3 = create(:position, stock: stock, portfolio: portfolio, transaction_price: 20, current_price: price3, action: :buy, volume: 200, position_group: pg)
  expect(pg.average_price).to eql 11.25
end

这是我运行时的结果：

1) PositionGroup methods should calculate the Weighted Average Purchase Price of all positions
     Failure/Error: expect(pg.average_price).to eql 11.25

       expected: 11.25
            got: 0.0

       (compared using eql?)

我很确定问题出在这一行，来自我在 PositionGroup 上的回调方法 update_average_price:

avg_price = positions.sum("transaction_price * (volume/#{total_volume})")

是否有更好的方法来解决这个问题，或者是否有其他东西在我不应该的时候给了我 0.0？

Answer 1

avg_price = positions.sum("transaction_price * (volume/#{total_volume.to_f})")

to_f 缺失，正在转换为浮点数以获取小数。

示例

irb(main):022:0> Position.all
  Position Load (0.3ms)  SELECT "positions".* FROM "positions" LIMIT ?  [["LIMIT", 11]]
=> #<ActiveRecord::Relation [#<Position id: 1, transaction_price: 0.5e2, volume: 150, position_group_id: 1>, #<Position id: 2, transaction_price: 0.1e1, volume: 50, position_group_id: 1>]>
irb(main):023:0> Position.all.sum("transaction_price * (volume/#{total_volume.to_f})")
   (0.2ms)  SELECT SUM(transaction_price * (volume/200.0)) FROM "positions"
=> 37.75

有没有一种简单的方法可以计算 Rails 中的和积？

Is there a simple way to calculate sumproduct in Rails?

ruby-on-rails

ruby-on-rails-5